Corynebacterium spp.

Other gram-negative rods

2)Extravascular Infections:

Except for intravascular infections, bacteria usually enter the circulation through the lymphatic system. Most

cases of clinically significant bacteremia are a result of extravascular infection. When organisms multiply at a

local site of infection such as the lung, they are drained by the lymphatics and reach the bloodstream. In most

individuals, organisms in the bloodstream are effectively and rapidly removed by the reticuloendothelial system

in the liver, spleen, and bone marrow and by circulating phagocytic cells. Depending on the extent of

immunologic control of the infection, the organism may be circulated more widely, thereby causing a

bacteremia or fungemia..

The most common portals of entry for bacteremia are the genitourinary tract (25%), respiratory tract (20%),

abscesses (10%), surgical wound infections (5%), biliary tract (5%), miscellaneous sites (10%), and uncertain

sites (25%). For the most part, the probability of bacteremia occurring from an extravascular site depends on

the site of infection, its severity, and the organism. For example, any organism producing meningitis is likely to

produce bacteremia at the same time.

 Of importance, certain organisms causing extravascular infections commonly invade the bloodstream; In

addition to these organisms, a large number of other bacteria and fungi that cause extravascular infections are

also capable of invading the bloodstream. Whether these organisms invade the bloodstream depends on the

host’s ability to

control the infection and the organism’s pathogenic potential. Some of the organisms associated with potential

bloodstream infections from a localized site include members of the family Enterobacteriaceae, Streptococcus

pneumoniae, Staphylococcus aureus, Neisseria gonorrhoeae,

anaerobic cocci, Bacteroides, Clostridium, beta-hemolytic streptococci, and Pseudomonas. These are only

some of the organisms frequently isolated from blood. Almost every known bacterial species and many fungal

species have been implicated in extravascular bloodstream infection.

Clinical manifestations:

 bacteremia may indicate the presence of a focus of disease, such as intravascular infection, pneumonia, or

liver abscess, or it may represent transient release of bacteria into the bloodstream. Septicemia or sepsis

indicates a condition in which bacteria or their products (toxins) are causing harm to the host. Unfortunately,

clinicians often use the terms bacteremia

and septicemia interchangeably. Signs and symptoms of septicemia may include fever or hypothermia (low

body temperature), chills, hyperventilation (abnormally increased breathing leading to excess loss of carbon

dioxide from the body) and subsequent respiratory alkalosis (a condition caused by the loss of acid leading to

an increase in pH), skin lesions, change in mental status, and diarrhea. More serious manifestations include

hypotension or shock, DIC, and major organ system failure.

The syndrome known as septic shock, characterized by fever, acute respiratory distress, shock, renal failure,

intravascular coagulation, and tissue destruction, can be initiated by either exotoxins or endotoxins.

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 Septic shock is mediated by the production of cytokines from activated mononuclear cells, such as tumor

necrosis factor and interleukins Shock is the gravest complication of septicemia. In septic shock, the presence

of bacterial products and the host’s response act to shut down major host physiologic systems.

Clinical manifestations include a drop in blood pressure, increase in heart rate, functional impairment in vital

organs (brain, kidney, liver, and lungs), acid base alterations, and bleeding problems. Gram-negative bacteria

contain a substance in their cell walls, called endotoxin, which has a strong effect on several physiologic

functions. This substance, a lipopolysaccharide (LPS) comprising part of the cell wall structure , may be

released during the normal growth cycles of bacteria or after the destruction of bacteria by host defenses.

Endotoxin (or the core of the LPS, lipid A) has been shown to mediate numerous systemic reactions, including

a febrile response, and the activation of complement and certain blood-clotting factors. Although gram-positive

bacteria do not contain the lipid A endotoxin, many produce exotoxins, and the effects of their presence in the

bloodstream may be equally devastating to the patient.

Disseminated intravascular coagulation (DIC) is a complication of sepsis. DIC is characterized by numerous

small blood vessels becoming clogged with blood clots and bleeding as a result of the depletion of coagulation

factors. DIC can occur with septicemia involving any circulating pathogen, including parasites, viruses, and

fungi, although it is most often a consequence of gram-negative bacterial sepsis.

Immunocompromised patients:

One of the greatest challenges facing microbiologists is the handling of blood cultures from

immunocompromised patients. The number of immunocompromised patients has steadily increased in recent

years in large part as the result of advances in medicine. People undergoing organ transplantation, elderly

persons, individuals with malignant disease (e.g., malignancies and cancer),

and those receiving therapy for the malignancy are examples of immunosuppressed patients. Acquired

immunodeficiency syndrome (AIDS) has also contributed to the increase in the number of

immunocompromised individuals.

The marked immunosuppression brought about by infection with the human immunodeficiency virus

(HIV) in patients with AIDS is a result of this virus’ profound impairment of cellular immunity. Patients with

AIDS have the greatest diversity of pathogens recovered from blood, including mycobacterial species,

Bartonella henselae, Corynebacterium jeikeium, Shigella flexneri, unusual Salmonella species, Histoplasma

capsulatum, Cryptococcus neoformans, and cytomegalovirus.

As is typically observed in other hospitalized patients, organisms such as gram-positive aerobic bacteria (e.g.,

Staphylococcus aureus, Enterococcus) and gram-negative aerobic bacteria (e.g., Enterobacteriaceae,

Pseudomonas aeruginosa) are common causes of bloodstream infections in immunocompromised patients. In

addition, bloodstream infections in immunocompromised patients are frequently caused by either unusual

pathogens whose recovery from blood requires special techniques or by organisms normally considered

contaminants when isolated from blood cultures.

Therefore, microbiologists must be aware of the potential pathogenicity of organisms in immunosuppressed

patients that are typically considered as probable blood culture contaminants.

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Without this knowledge, aerobic gram-positive rods isolated from blood cultures may be dismissed as

contaminating diphtheroids, when, in fact, the organism is C. jeikeium, known to cause bacteremia in

immunosuppressed patients. Microbiologists must be familiar with the unusual pathogens isolated from blood

cultures obtained from immunocompromised patients and organisms that require special techniques for

isolation

Specimen collection:

Antisepsis. Once a vein is selected, the skin site is defatted (fat removal) with 70% isopropyl alcohol and an

antiseptic is applied to kill surface and subsurface bacteria. Regardless of the antiseptic used, it is critical to

follow the manufacturer’s recommendation for the length of time the antiseptic is allowed to remain on the

skin. Available data indicate that iodine tincture (iodine in alcohol) and chlorhexidine are equivalent for skin

preparation before drawing blood cultures.

Drawing Blood for Culture: Organisms found in circulating blood can be enriched in culture for isolation and

further studies. Blood for culture must be obtained aseptically. Once removed from the circulation, unclotted

blood must be diluted in growth media. Universal precautions require that phlebotomists wear gloves for this

procedure:

1. Choose the vein to be drawn by touching the skin before it has been disinfected.

2. Using 70% alcohol, cleanse the skin over the venipuncture site in a circle approximately 5 cm in diameter,

rubbing vigorously. Allow to air-dry.

3. Starting in the center of the circle, apply 2% tincture of iodine (or povidone-iodine) in ever-widening circles

until the entire circle has been saturated with iodine. Allow the iodine to dry on the skin for at least 1 minute.

The timing is critical; a watch or timer should be used.

4. If the phlebotomist must touch the site after preparation, the phlebotomist must disinfect the gloved fingers

used for palpation in identical fashion.

5. Insert the needle into the vein and withdraw blood. Do not change needles before injecting the blood into the

culture bottle.

6. After the needle has been removed, the site should be cleansed with 70% alcohol again, because many

patients are sensitive to iodine.

Specimen Volume:

Adults. There is a direct relationship between the volume of blood and an increased probability that the

laboratory will isolate the infecting the organism. Therefore, collection of two sets of cultures using10 to 20 mL

of blood per culture is strongly recommended for adults.

Children. For infants and small children, only 1 to 5 mL of blood should be drawn for bacterial culture. Blood

culture bottles are available designed specifically for the pediatric patient. Because blood specimens from septic

children may yield fewer than 5 CFU/mL of the organism, quantities less than 1 mL may not be adequate to

detect pathogens. Nevertheless, smaller volumes should still be cultured because high levels of bacteremia

(more than 1000 CFU/mL of blood) are detected in some infants.

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Number of Blood Cultures:usually two or three blood cultures are sufficient to achieve the optimum blood

culture sensitivity.

Timing of Collection:

The timing of cultures is not as important as other factors in patients with intravascular infections because

organisms are released into the bloodstream at a fairly constant rate. Because the timing of intermittent

bacteremia is unpredictable, it is generally accepted that two or three blood cultures be spaced an hour apart.

blood should be transported immediately to the laboratory and placed into the incubator or instrument as soon

as possible. With blood culture instrumentation, a delay beyond 2 hours can delay the detection of positive

cultures.

Miscellaneous Matters:

Anticoagulation. Blood drawn for culture must not be allowed to clot. If bacteria become entrapped within a

clot, their presence may go undetected. Thus, blood drawn for culture may be either inoculated directly into the

blood culture broth media .

Heparin, ethylenediaminetetraacetic acid (EDTA), and citrate inhibit numerous organisms and are not

recommended for use. Sodium polyanethol sulfonate (SPS, Liquoid) in concentrations of 0.025% to 0.03% is

the best anticoagulant available for blood cultures. As a result, the most commonly used preparation in blood

culture media today is 0.025% to 0.05% SPS. In addition to its anticoagulant properties, SPS is also

anticomplementary and antiphagocytic, and interferes with the

activity of some antimicrobial agents, notably aminoglycosides.

SPS, however, may inhibit the growth of a few microorganisms, such as some strains of Neisseria spp.,

Gardnerella vaginalis, Streptobacillus moniliformis, and all strains of Peptostreptococcus anaerobius.

Blood Culture Media: The diversity of bacteria recovered from blood requires an equally diverse and large

number of media to enhance the growth of these bacteria. Basic blood culture media contain a nutrient broth

and an anticoagulant. Several different broth formulations are commercially available. Most blood culture

bottles available commercially contain trypticase soy broth, brain-heart infusion broth, supplemented peptone,

or thioglycolate broth. More specialized broth bases include Columbia or Brucella broth.

 BacT/ALERT has a blood culture bottle with supplemented brain heart infusion (BHI) broth containing

activated charcoal particles that significantly increase the yield of microorganisms over standard blood culture

media. In addition, resins or charcoal may be added to commercial media to absorb and inactivate antimicrobial

agents within the patient’s blood.

Special blood culture broth systems are available for the isolation of mycobacteria. The systems are useful in

detecting disseminated infections caused by Mycobacterium tuberculosis and non-tuberculosis mycobacteria.

To subculture, the procedure that does not require opening the bottle, the large volume of broth subcultured and

the enclosed method provide faster detection for many organisms than is possible with conventional systems

Incubation Conditions. The atmosphere in commercially prepared blood culture bottles is usually permitting

the growth of most facultative

and some anaerobic organisms. To encourage the growth of obligate (strict) aerobes, such as yeast and

Pseudomonas aeruginosa, transient venting of the bottles with a sterile, cotton-plugged needle may be

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necessary. Constant agitation of the bottles during the first 24 hours of incubation also enhances the growth of

most aerobic bacteria.

BACTEC Systems: Many laboratories use the BACTEC system (Becton Dickinson Microbiology Systems,

Sparks, Maryland), which measures the production of carbon dioxide (CO2) by metabolizing organisms. Blood

or sterile body fluid for routine culture is inoculated into bottles containing appropriate substrates.

The first BACTEC systems were semiautomated. Vials, containing 14C-labeled substrates (glucose, amino

acids) were incubated and often agitated on a rotary shaker. At predetermined time intervals thereafter, the

bottles were placed into the monitoring module, where they were automatically moved to a detector. The

detector inserted two needles through a rubber septum seal at the top of each bottle and withdrew the

accumulated gas above the liquid medium and replaced it with fresh gas of the same mixture (aerobic or

anaerobic). Any amount of radiolabeled CO2, the final end product of metabolism of the 14C-labeled substrates

(above a preset baseline level), was considered to be suspicious for microbial growth. Microbiologists retrieved

suspicious bottles and worked them up (performed subcultured and identification procedures) for possible

microbial growth.

 Other laboratories use the BacT/Alert System (bioMérieux, Durham, North Carolina), which measures CO2-

derived pH changes with a colorimetric sensor in the bottom of each bottle (see Figure 3). The sensor is

separated from the broth medium by a membrane permeable to CO2. As organisms grow, they release CO2,

which diffuses across

the membrane and is dissolved in water present in the matrix of the sensor. As CO2 is dissolved, free hydrogen

ions are generated. These free hydrogen ions cause a color change in the sensor (blue to light green to yellow as

the pH decreases); a sensor in the instrument reads this color change.

Handling Positive Blood Cultures:

Most laboratories use a broth-based automated blood culture method. When a positive culture is indicated

according to the automated detection system, or manual technique: a Gram stained smear of an air-dried drop of

Figure 3 A, Blood culture bottles for the BACTEC 9240, 9120, and 9050 continuous

monitoring instruments. B, The BD BACTEC FX continuous monitoring blood culture

system

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medium should be performed. Methanol fixation of the smear preserves bacterial and cellular morphology,

which may be especially

valuable for detecting gram-negative bacteria among red cell debris. Designed to maximize sensitivity,

detection algorithms of automated blood culture instruments lead to a certain percentage of false-positive

results. Thus, in addition to performance of a Gram stain using methanol fixation , As soon as a morphologic

description can be tentatively assigned to an organism detected in blood, the physician should be contacted and

given all available information.

Determining the clinical significance of an isolate is the physician’s responsibility. If no organisms are seen on

microscopic examination of a bottle that appears positive, subcultures should be performed anyway.

Subcultures from blood cultures suspected of being positive, whether proved by microscopic visualization or

not, should be made to various media that would support the growth of most bacteria, including anaerobes.

Initial subculture may include chocolate agar, 5% sheep blood agar, MacConkey agar, and supplemented

anaerobic blood agar.

 In addition, some laboratories are subculturing to specialized chromogenic agar for the isolation of specific

pathogenic organisms such as MNSA, yeast (Candida spp.). The incidence of polymicrobial bacteremia or

fungemia ranges from 3% to 20% of all positive blood cultures. For this reason, samples must be resubcultured

for isolated colonies.

Numerous rapid tests for identification and presumptive antimicrobial susceptibilities can be performed from

the broth blood culture if a monomicrobic infection is suspected (based on microscopic evaluation). A

suspension of the organism that approximates the turbidity of a 0.5 McFarland standard, obtained directly from

the broth or by centrifuging the broth and resuspending the pelleted bacteria, can be used to perform either disk

diffusion (qualitative) or broth dilution (quantitative) antimicrobial susceptibility tests. These suspensions may

also be used to perform preliminary tests such as coagulase, thermostable nuclease, esculin hydrolysis, bile

solubility, antigen detection by fluorescent-antibody stain or agglutination procedures for gram-positive

bacteria, oxidase, and commercially available rapid identification kits for

gram-negative bacteria. Presumptive results must be verified with conventional procedures using pure cultures.

In addition to these approaches, the introduction of a number of molecular methods, including conventional and

peptide nucleic acid hybridization assays using specific probes, conventional and real-time polymerase chain

reaction assays and microarrays have been used to directly identify microorganisms in blood culture bottles.

 In the event of possible future studies (e.g., additional susceptibility testing), all isolates from blood cultures

should be stored for a minimum of 6 months by freezing . It is often necessary to compare separate isolates

from the same patient or isolates of the same species from different patients months.

 On one hand, contaminants may lead to unnecessary antibiotic therapy, additional testing and consultation,

and increased length of hospital stay. Costs related to false-positive blood culture results (i.e., contaminants) are

associated with 40% higher charges for IV antibiotics and microbiology testing. On the other hand, failure to

recognize and appropriately treat indigenous microflora can have dire consequences.

Guidelines that can assist in distinguishing probable pathogens from contaminants are as follows:

 Probable contaminant

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• Growth of Bacillus spp., Corynebacterium spp., Propionibacterium

acnes, or coagulase-negative staphylococci in one of several cultures

Note: Bacillus anthracis must be ruled out before dismissing Bacillus species as a probable contaminant.

• Growth of multiple organisms from one of several cultures (polymicrobial bacteremia is uncommon)

• The clinical presentation or course is not consistent with sepsis (physician-based, not laboratory-based

criteria)

• The organism causing the infection at a primary site of infection is not the same as that isolated from the

blood culture.

 Probable pathogen

• Growth of the same organism in repeated cultures obtained either at different times or from different anatomic

sites

• Growth of certain organisms in cultures obtained from patients suspected of endocarditis, such as enterococci,

or gram-negative rods in patients with clinical gram-negative sepsis

• Growth of certain organisms such as members of Enterobacteriaceae, Streptococcus pneumoniae,

gramnegative anaerobes, and Streptococcus pyogenes

• Isolation of commensal microbial flora from blood cultures obtained from patients suspected to be bacteremic

(e.g., immunosuppressed patients or those having prosthetic devices).

Normally Sterile Body Fluids, Bone and Bone Marrow, and Solid Tissues

The human body is divided into five main body cavities: cranial, spinal, thoracic, abdominal, and pelvic. Each

cavity is lined with membranes, and within the body wall and these membranes, or between the membranes and

organs, are small spaces filled with minute amounts of fluid. The purpose of this fluid is to bathe the organs and

membranes, reducing the friction between organs.

Bacteria, fungi, virus, or parasite can invade any body tissue or sterile body fluid site. Although from different

areas of the body, all specimens discussed in this chapter are considered normally sterile. Therefore, even one

colony of a potentially pathogenic microorganism may be significant.

Specimens from sterile body sites:

FLUIDS :In response to infection, fluid may accumulate in any body cavity. Infected solid tissue often presents

as cellulitis or with abscess formation. Areas of the body from which fluids are typically sent for microbiologic

studies include those in ( Table -1.)

Pleural Fluid : Lining the entire thoracic cavity of the body is a serous membrane called the parietal pleura.

Covering the outer surface of the lung is another membrane called the visceral pleura . Within the pleural space

between the lung and chest wall is a small amount of fluid called pleural fluid that lubricates the surfaces of the

pleura (the membranes surrounding the lungs and lining the chest cavity). Normally, equilibrium exists among

the pleural membranes, but in certain disease states, such as cardiac, hepatic, or renal disease, excess amounts

of this fluid can be produced and accumulates in the pleural space; this is known as a pleural effusion. Pleural

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effusions can either be exudative or transudative. Exudative pleural effusions are caused by inflammation,

infection, and cancer, whereas transudative effusions are due to systemic changes, such as congestive heart

failure.

Normal pleural fluid contains few or no cells and has a consistency similar to serum, but with a lower protein

count. Pleural fluid containing numerous white blood cells is indicative of infections. Pleural fluid specimens

are collected by thoracentesis, a procedure in which a needle is inserted through the chest wall into the pleural

space and the excess fluid aspirated. This fluid is then submitted to the laboratory as thoracentesis fluid, pleural

fluid, or empyema fluid. The fluid, or effusion, can then

be analyzed for cell count, total protein, glucose, lactate dehydrogenase, amylase, cytology, and culture.

 The total protein and glucose results determine if the effusion is transudate or exudate. The patient’s serum or

plasma glucose level is needed to compare with the results indicated in the body fluid. Several characteristics

can be used to determine whether a fluid is a transudate or

exudate (Table 2).

 When effusions are extremely purulent or full of pus, the effusion is referred to as an empyema. Empyema

often arises as a complication of

pneumonia, but other infections near the lung (e.g., subdiaphragmatic infection) may seed microorganisms into

the pleural cavity. It has been estimated that 50% to 60% of patients develop empyema as a complication of

pneumonia.

Peritoneal Fluid:

The peritoneum is a large, moist, continuous sheet of serous membrane lining the walls of the abdominalpelvic

cavity and the outer coat of the organs contained within the cavity . In the abdomen, these two membrane

linings are separated by a space called the peritoneal cavity, which contains or abuts the liver pancreas, spleen,

stomach and intestinal tract, bladder, and fallopian tubes and ovaries. The kidneys occupy a retroperitoneal

(behind the peritoneum) position. Within the healthy human peritoneal cavity is a small amount of fluid that

maintains the surface moisture of the peritoneum. Normal peritoneal fluid may contain as many as 300 white

blood cells per milliliter, but the protein content and specific gravity of the fluid are low.

During an infectious or inflammatory process, increased amounts of fluid accumulate in the peritoneal cavity, a

condition called ascites. Most cases of ascites are due to liver disease, and in severe cases, the abdomen is often

distended. The fluid can be collected for testing by paracentesis (the insertion of a needle into the abdomen and

removal of fluid). The peritoneal or ascites fluid can then be analyzed for amylase, protein, albumin, cell count,

culture, and cytology. Often ascitic fluid contains

an increased number of inflammatory cells and an elevated protein level.

Agents of infection gain access to the peritoneum through a perforation of the bowel, through infection within

abdominal viscera, by way of the bloodstream, or by external inoculation (as in surgery or trauma). On

occasion, as in pelvic inflammatory disease (PID)

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(Table-1 ):Microbiology Laboratory Body Fluid Collection Sites:

Body Area Fluid Name(s)

Thorax Thoracentesis or pleural or empyema fluid

Abdominal cavity Paracentesis or ascitic or peritoneal fluid

Joint Synovial fluid

Pericardium Pericardial fluid

organisms travel through the natural channels of the fallopian tubes into the peritoneal cavity.

Primary Peritonitis. Peritonitis results when the peritoneal membrane becomes inflamed and can be either

primary or secondary. Primary peritonitis is rare and results when infection spreads from the blood and lymph

nodes with no apparent evidence of infection. The organisms likely to be recovered from patient specimens.

 (Table 2): Pleural Fluid Effusion Characteristics:

Transudate Exudate

Appearance Clear Cloudy

Specific Gravity <1.015 >1.015

Total Protein <3.0 mg/dL >3.0 mg/dL

LD Fluid: Serum Ratio <0.6 >0.6

Cholesterol <60 mg/dL >60 mg/dL

Cholesterol Fluid: <0.3 >0.3

Serum Ratio

Bilirubin Fluid:Serum <0.6 >0.6

Ratio

Total Protein Fluid: <0.5

Serum Ratio

<1000/μL (all white blood >1000/μ

cell types, all <50%)

White Blood Cells

<10,000/μL = because of >100,000/μL

traumatic tap

Red Blood Cells

Clotting Will not clot May clot

with primary peritonitis vary with the patient’s age. The most common etiologic agents in children are

Streptococcus pneumoniae and group A streptococci, Enterobacteriaceae, other gram-negative bacilli, and

staphylococci. In adults, Escherichia coli is the most common bacterium, followed by S. pneumoniae and group

A streptococci. Polymicrobic peritonitis is unusual in the absence of bowel perforation or rupture. Among

sexually active young women, Neisseria gonorrhoeae and Chlamydia trachomatis are common etiologic agents

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of peritoneal infection, often in the form of a perihepatitis (inflammation of the surface of the liver, called FitzHugh–Curtis syndrome).

Fungal causes of peritonitis are not common, but Candida spp. may be recovered from immunosuppressed

patients and patients receiving prolonged antibacterial therapy.

Secondary Peritonitis. Secondary peritonitis is a complication of a perforated viscus (organ), surgery,

traumatic injury, loss of bowel wall integrity following a destructive disease (e.g., ulcerative colitis, ruptured

appendix, carcinoma), obstruction, or a preceding infection (liver abscess, salpingitis, septicemia). The nature,

location, and etiology of the underlying process govern the agents recovered from peritoneal fluid. With PID as

the background, gonococci, anaerobes, or chlamydiae are isolated. With peritonitis or intra-abdominal abscess,

anaerobes generally are found in peritoneal fluid, usually together with Enterobacteriaceae and enterococci or

other streptococci. In patients whose bowel flora has

been altered by antimicrobial agents, more resistant gram-negative bacilli and Staphylococcus aureus may be

encountered. Because anaerobes outnumber aerobes in the bowel by 1000-fold, it is not surprising that

anaerobic organisms play a prominent role in intra-abdominal infection, perhaps acting synergistically with

facultative bacteria. The organisms likely to be recovered include E.coli, the Bacteroides fragilis group,

enterococci and other streptococci, Bilophila spp., other anaerobic gramnegative bacilli, anaerobic grampositive cocci, and clostridia.

Peritoneal Dialysis Fluid:

Continuous ambulatory peritoneal dialysis (CAPD). the number of organisms is usually too low for detection

on Gram stain of the peritoneal fluid sediment unless a concentrating technique is used; fungi are more readily

detected. Many recent studies show that improved sensitivity can be achieved by using automated blood culture

systems in which 10 mL of fluid is inoculated into culture bottles.

Most infections originate from the patient’s own skin flora; Staphylococcus epidermidis and S. aureus are the

most common etiologic agents, followed by streptococci, aerobic or facultative gram-negative bacilli, Candida

spp., Corynebacterium spp., and others. The oxygen content of peritoneal dialysate is usually too high for the

development of anaerobic infection. Among the gram-negative bacilli isolated, Pseudomonas spp.,

Acinetobacter spp., and the Enterobacteriaceae are frequently observed.

Pericardial Fluid:

The heart and contiguous major blood vessels are surrounded by the pericardium, a protective tissue. The area

between the epicardium, which is the membrane surrounding the heart muscle, and the pericardium is

called the pericardial space and normally contains 15 to 20 mL of clear fluid. If an infectious agent is present

within the fluid, the pericardium may become distended and tight, and eventually tamponade (interference with

cardiac function and circulation) can ensue. Up to 500 mL of fluid can accumulate during infection, which may

seriously complicate cardiac function.

Agents of pericarditis (inflammation of the pericardium) are usually viruses, especially Coxsackie virus.

Parasites, bacteria, certain fungi, and noninfectious causes are also associated with this disease.

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Myocarditis (inflammation of the heart muscle itself) may accompany or follow pericarditis. The pathogenesis

of disease involves the host inflammatory response contributing to fluid buildup as well as cell and tissue

damage.

Common causes of myocarditis include viral infections with Coxsackie virus, echoviruses, or adenovirus. The

most common etiologic agents of pericarditis and myocarditis are listed in Table 3. Other bacteria, fungi, and

parasitic agents have been recovered from pericardial effusions.

Patients who develop pericarditis resulting from agents other than viruses are often immunocompromised or

suffering from a chronic disease. An example is infective endocarditis, in which a myocardial abscess develops

and then ruptures into the pericardial space.

Joint Fluid Arthritis is an inflammation in a joint space. Infectious arthritis may involve any joint in the body.

Infection of the joint usually occurs secondary to hematogenous spread of bacteria or, less often, fungi, as a

direct extension of infection of the bone. It may also occur after

injection of material, especially corticosteroids, into joints or after insertion of prosthetic material (e.g., total hip

replacement). Although infectious arthritis usually occurs at a single site (monoarticular), a preexisting

bacteremia or fungemia may seed more than one joint to establish polyarticular infection, particularly when

multiple joints are diseased, such as in rheumatoid arthritis.

In bacterial arthritis, the knees and hips are the most commonly affected joints in all age groups.

In addition to active infections associated with viable microorganisms within the joint, sterile, self-limited

arthritis caused by antigen-antibody interactions may follow an episode of infection, such as meningococcal

meningitis. When an etiologic agent cannot be isolated from an inflamed joint fluid specimen, either the

absence of viable agents or inadequate transport or culturing procedures may be the cause. For example, even

under the best circumstances, Borrelia burgdorferi is isolated from the joints of fewer than 20% of patients

with Lyme disease. Nonspecific test results, such as increased white blood cell count, decreased glucose, or

elevated protein, may indicate that an infectious agent is present but inconclusive.

Overall, Staphylococcus aureus is the most common etiologic agent of septic arthritis, accounting for

approximately 70% of infections. In adults younger than 30 years of age, however, Neisseria gonorrhoeae is

isolated most frequently. Haemophilus influenzae has been the most common agent of bacteremia in children

younger than 2 years of age, and consequently it has been the most frequent cause of infectious arthritis in these

patients, followed by S. aureus. The widespread use of H. influenza type B vaccine should contribute to a

change in this pattern. Streptococci, including groups A (Streptococcus pyogenes) and B (Streptococcus

agalactiae), pneumococci, and viridans streptococci, are prominent among bacterial agents associated with

infectious arthritis in patients

of all ages. Among anaerobic bacteria, Bacteroides, including B. fragilis, may be recovered and Fusobacterium

necrophorum, which usually involves more than one joint in the course of sepsis. Among people living in

certain endemic areas of the United States and Europe,

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(Table 3) :Common Etiologic Agents of Pericarditis and Myocarditis:

Viruses:

Enteroviruses (primary Coxsackie A and B and, less

frequently, echoviruses)

Adenoviruses

Influenza viruses

Bacteria (relatively uncommon):

Mycoplasma pneumoniae

Chlamydia trachomatis

Mycobacterium tuberculosis

Staphylococcus aureus

Streptococcus pneumoniae

Enterobacteriaceae and other gram-negative bacilli

Fungi (relatively uncommon):

Coccidioides immitis

Aspergillus spp.

Candida spp.

Cryptococcus neoformans

Histoplasma capsulatum

Parasites (relatively uncommon):

Entamoeba histolytica

Toxoplasma gondii

(Table 4): Most Frequently Encountered Etiologic Agents of Infectious Arthritis:

Bacterial:

Staphylococcus aureus

Beta-hemolytic streptococci

Streptococci (other)

Haemophilus influenzae

Haemophilus spp. (other)

Bacteroides spp.

Fusobacterium spp.

Neisseria gonorrhoeae

Pseudomonas spp.

Salmonella spp.

Pasteurella multocida

Moraxella osloensis

Kingella kingae

188

 Arranged by Sarah Mohssen

Section I– Microbiology By Nada Sajet

Moraxella catarrhalis

Capnocytophaga spp.

Corynebacterium spp.

Clostridium spp.

Peptostreptococcus spp.

Eikenella corrodens

Actinomyces spp.

Mycobacterium spp.

Mycoplasma spp.

Ureaplasma urealyticum

Borrelia burgdorferi

Fungal:

Candida spp.

Cryptococcus neoformans

Coccidioides immitis

Sporothrix schenckii

Viral:

Hepatitis B

Mumps

Rubella

Other viruses (rarely)

infectious arthritis is a prominent feature associated with Lyme disease. Chronic monoarticular arthritis is

frequently due to mycobacteria, Nocardia asteroides, and fungi. Some of the more frequently encountered

etiologic agents of infectious arthritis are listed in (Table 4)

These agents act to stimulate a host inflammatory response, which is initially responsible for the pathology of

the infection. Arthritis is also a symptom associated with infectious diseases caused by certain agents, such as

Neisseria meningitidis, group A streptococci (rheumatic fever), and Streptobacillus moniliformis, in which the

agent cannot be recovered from joint fluid.

 Presumably, antigen-antibody complexes formed during active infection accumulate in a joint, initiating an

inflammatory response that is responsible for the ensuing damage.

Infections in prosthetic joints are usually associated with somewhat different etiologic agents than those in

natural joints. After insertion of the prosthesis, organisms that gained access during the surgical procedure

slowly multiply until they reach a critical mass and produce a host response. This may occur long after the

initial surgery; approximately half of all prosthetic joint infections occur more than 1 year after surgery. Skin

flora is the most common etiologic agent, with Staphylococcus epidermidis, other coagulase-negative

staphylococci, Corynebacterium spp., and Propionibacterium spp. as the most common. However,