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mirabilis, P. penneri, and P. vulgaris “swarm” on blood and chocolate agars. Swarming results in the

production of a thin film of growth on the agar surface (Figure 3) as the motile organisms spread from the

original site of inoculation. Colonies of Y. pestis on 5% sheep blood agar are pinpoint at 24 hours but exhibit a

rough, cauliflower appearance at 48 hours. Broth cultures of Y. pestis exhibit a characteristic “stalactite pattern”

in which clumps of cells adhere to one side of the tube.

Y. enterocolitica produces bull’s-eye colonies (dark red or burgundy centers surrounded by a translucent

border; see Figure 1) on CIN agar at 48 hours. However, because most Aeromonas spp. produce similar

colonies on

CIN agar, it is important to perform an oxidase test to verify that the organisms are Yersinia spp. (oxidase

negative).

The oxidase test should be performed on suspect colonies that have been subcultured to sheep blood agar.

Pigments present in the CIN agar will interfere with correct interpretation of the oxidase test results.

Table (4) Colonial Appearance and Characteristics of the Most Commonly Isolated Enterobacteriaceae

Organism Medium Appearance

Citrobacter spp. MAC Late LF; therefore, NLF after 24 hr; LF after 48 hr; colonies are light pink after 48

hr

MAC Colorless

Edwardsiella spp. MAC NLF

HE Colorless

XLD Red, yellow, or colorless colonies, with or without black centers (H2S)

Enterobacter spp. MAC LF; may be mucoid

HE Yellow

XLD Yellow

Escherichia coli MAC Most LF, some NLF (some isolates may demonstrate slow or late fermentation);

and generally flat, dry, pink colonies with a surrounding darker pink area of

precipitated bile salts†

HE Yellow

XLD Yellow

Hafnia alvei MAC NLF

HE Colorless

XLD Red or yellow

Klebsiella spp. MAC LF; mucoid

HE Yellow

XLD Yellow

Morganella spp. MAC NLF

HE Colorless

XLD Red or colorless

Plesiomonas BAP Shiny, opaque, smooth, nonhemolytic

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shigelloides

MAC Can be NLF or LF

Proteus spp. MAC NLF; may swarm, depending on the amount of agar in the medium; characteristic

foul smell

HE Colorless

XLD Yellow or colorless, with or without black centers

Providencia spp. MAC NLF

HE Colorless

XLD Yellow or colorless

Salmonella spp. MAC NLF

HE Green, black center as a result of H2S production

XLD Red with black center

Serratia spp. MAC Late LF; S. marcescens may be red pigmented, especially if plate is left at 25°C

(Figure 20-2)

HE Colorless

XLD Yellow or colorless

Shigella spp. MAC NLF; S. sonnei produces flat colonies with jagged edges

HE Green

XLD Colorless

Yersinia spp. MAC NLF; may be colorless to peach

HE Salmon

XLD Yellow or colorless

HE, Hektoen enteric agar; LF, lactose fermenter, pink colony; MAC, MacConkey agar; NLF, non–lactose

fermenter, colorless colony; XLD, xylose-lysinedeoxycholate agar.

*Most Enterobacteriaceae are indistinguishable on blood agar. Pink colonies on MacConkey agar with sorbitol

are sorbitol fermenters; colorless colonies are non–sorbitol fermenters.

Approach to identification

In the early decades of the twentieth century, Enterobacteriaceae were identified using more than 50

biochemical tests in tubes; this method is still used today in reference and public health laboratories. Certain

key tests such as indole, methyl red, Voges-Proskauer, and citrate, known by the acronym IMViC, were

routinely performed to group the most commonly isolated pathogens.

Today, this type of conventional biochemical identification of enterics has become a historical footnote in most

clinical and hospital laboratories in the

United States.

In the latter part of the twentieth century, manufacturers began to produce panels of miniaturized tests for

identification, first of enteric gram-negative rods and later of other groups of bacteria and yeast. Original panels

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were inoculated manually; these were followed by semiautomated and automated systems, the most

sophisticated of which inoculate, incubate, read, and discard

the panels. Practically any commercial identification system can be used to reliably identify the commonly

isolated Enterobacteriaceae. Depending on the system, results are available within 4 hours or after overnight

incubation. The extensive computer databases used by these systems include information on unusual biotypes.

The number of organisms used to define individual databases is important; in rare cases, isolated organisms or

new microorganisms may be misidentified or not identified at all.

The definitive identification of enterics can be enhanced based on molecular methods, especially 16S ribosomal

RNA (rRNA) sequencing and DNA-DNA

hybridization. Through the use of molecular methods, the genus Plesiomonas, composed of one species of

oxidase-positive, gram-negative rods, now has been

included in the family Enterobacteriaceae. Plesiomonas sp. clusters with the genus Proteus in the

Enterobacteriaceae by 16S rRNA sequencing. However, like all other Enterobacteriaceae, Proteus organisms

are Oxidase negative.

The clustering together of an oxidase-positive genus and an oxidase-negative genus is a revolutionary concept

in microbial taxonomy.

In the interests of cost containment, many clinical laboratories use an abbreviated scheme to identify commonly

isolated enterics. E. coli, for example, the most commonly isolated enteric organism, may be identified by a

positive spot indole test For presumptive identification of an organism as E. coli, the characteristic colonial

appearance on MacConkey agar, as described in( Table 4), is documented along with positive spot indole test

result. A spot indole test can alsobe used to quickly separate swarming Proteae, such as P. mirabilis and P.

penneri, which are negative, from the indole-positive P. vulgaris.

Figure( 2) Red-pigmented Serratia marcescens on MacConkey agar.

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Specific Considerations for Identifying Enteric Pathogens

 The common biochemical tests used to differentiate the species in the genus Citrobacter are illustrated in

(Table 3.)

 In most clinical laboratories, serotyping of Enterobacteriaceae is limited to the preliminary grouping of

Salmonella spp., Shigella spp., and E. coli O157:H7. Typing should be performed from a non–sugar-containing

medium, such as 5% sheep blood agar or LIA. Use of sugar-containing media, such as MacConkey or TSI

agars, can cause the organisms to autoagglutinate.

Commercially available polyvalent antisera designated A, B, C1, C2, D, E, and Vi are commonly used to

preliminarily group Salmonella spp. because 95% of isolates belong to groups A through E. The antisera A

through E contain antibodies against somatic (“O”) antigens, and the Vi antiserum is prepared against the

capsular (“K”) antigen of S. serotype Typhi. Typing is performed using a slide agglutination test. If an isolate

agglutinates with the Vi antiserum and does not react with any of the “O” groups, then a saline suspension of

the organism should be prepared and heated to 100°C for 10 minutes to inactivate

the Vi antigen. The organism should then be retested. S. typhi is positive with Vi and group D. Complete typing

of Salmonella spp., including the use of antisera against the flagellar (“H”) antigens, is performed at reference

laboratories. Preliminary serologic grouping of Shigella spp. is also performed using commercially available

polyvalent somatic (“O”) antisera designated A, B, C, and D. As with Salmonella spp., Shigella spp. may

produce a capsule and therefore heating may be required before typing is successful. Subtyping of Shigella spp.

beyond the groups A, B, and C (Shigella group D only has one serotype) is typically performed in reference

laboratories.

P. shigelloides, a new member of the Enterobacteriaceae that can cause gastrointestinal infections might crossreact with Shigella grouping antisera, particularly group D, and lead to misidentification. This mistake can be

avoided by performing an oxidase test. Sorbitol-negative E. coli can be serotyped using commercially available

antisera to determine whether the somatic “O” antigen 157 and the flagellar “H” antigen 7 are present. Latex

Figure( 3) Proteus mirabilis swarming on blood agar (arrow points to swarming edge).

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reagents and antisera are now also available for detecting some non-0157, sorbitol-fermenting, Shiga toxin–

producing strains of E. coli.

Some national reference laboratories are simply performing tests for Shiga toxin rather than searchingfor O157

or non-O157 strains by culture. Unfortunately, isolates are not available then for strain typing for epidemiologic

purposes. Laboratory tests to identify enteropathogenic, enterotoxigenic, enteroinvasive, and enteroaggregative

E. coli that cause gastrointestinal infections usually involve animal, tissue culture, or molecular studies

performed in reference laboratories.

The current recommendation for the diagnosis of Shiga toxin–producing E. coli includes testing all stools

submitted from patients with acute community-acquired diarrhea to detect enteric pathogens (Salmonella,

Shigella, and Campylobacter spp.) should be cultured for O157 STEC on selective and differential agar. In

addition, these stools should be tested using either a Shiga toxin detection assay or a molecular assay to

simultaneously determine whether the sample contains a non-O157 STEC. To save media, some laboratories

may elect to perform the assay first, then attempt to grow organisms from broths with an assay-positive result

on selective media. In any case, any isolate or broth positive for 0157STEC, non- 0157STEC, or shiga toxin

should be forwarded to the public health laboratory for confirmation and direct immunoassay testing. Any

isolate positive for O157 STEC should be forwarded to the public health laboratory for additional

epidemiologic analysis. Any specimens or enrichment broths that are positive for Shiga toxin or

STEC but negative for O157 STEC should also be forward to the public health laboratory for further testing.

Most commercial systems can identify Y. pestis if a heavy inoculum is used. All isolates biochemically grouped

as a Yersinia sp. should be reported to the public health laboratory. Y. pestis should always be reported and

confirmed.

Serodiagnosis

 Serodiagnostic techniques are used for only two members of the family Enterobacteriaceae; that is, S. typhi

and Y.pestis. Agglutinating antibodies can be measured in the diagnosis of typhoid fever; a serologic test for S.

typhi is

part of the “febrile agglutinins” panel and is individually known as the Widal test. Because results obtained by

using the Widal test are somewhat unreliable, this method is no longer widely used.

Serologic diagnosis of plague is possible using either a passive hemagglutination test or enzyme-linked

immunosorbent assay; these tests are usually performed in reference laboratories.

Antimicrobial susceptibility testing and therapy:

 For many of the gastrointestinal infections caused by Enterobacteriaceae, inclusion of antimicrobial agents as

part of the therapeutic strategy is controversial or at least uncertain The unpredictable nature of any clinical

isolate’s antimicrobial susceptibility requires that testing be done as a guide to therapy. several standard

methods and commercial systems have been developed for this purpose.

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The Clinical and Laboratory

Standards Institute and (CLSI) has created guidelines (CLISI document M-100 and M100-S23) for the

minimum inhibitory concentration (MIC) and disk diffusion breakpoints for aztreonam, cefotaxime,

cefpodoxime, ceftazidime, and ceftriaxone for E. coli, Proteus, and Klebsiella spp., as well as for cefpodoxime,

ceftazidime, and cefotaxime for P. mirabilis. The sensitivity of the screening increases with the use of more

than a single drug. ESBLs are inhibited by clavulanic acid; therefore, this property can be used as a

confirmatory test in the identification process. In addition, with regard to cases

in which moxalactam, cefonicid, cefamandole, or cefoperazone is being considered to treat infection caused by

E. coli, Klebsiella spp., or Proteus spp., it is important to note that interpretive guidelines have not been

evaluated, and ESBL testing should be performed. If isolates test ESBL positive, the results of the antibiotics

listed should be reported as resistant.

 CLSI has revised the interpretive criteria for cephalosporins (cefazolin, cefotaxime, ceftazidime, ceftizoxime,

and ceftriaxone) and aztreonam. Using the new interpretive guidelines, routine ESBL testing is no longer

necessary, and it is no longer necessary to edit results for cephalosporins, aztreonam, or penicillins from

susceptible to resistant. ESBL testing will remain useful for epidemiologic and infection control purposes.

Mmultidrug-resistant typhoid fever (MDRTF)

Multidrug-resistant typhoid fever is caused by S. serotype Typhi strains resistant to chloramphenicol,

ampicillin, and cotrimoxazole. Isolates classified as MDRTF have been indentified since the early 1990s in

patients of all ages. The risk for the development of MDRTF is associated with the overuse, misuse and

inappropriate use of antibiotic therapy.

Susceptibility tests should be performed using the typical first-line antibiotics, including chloramphenicol,

ampicillin, and trimethoprimsulfamethoxazole, along with a fluoroquinolone and a nalidixic acid (to detect

reduced susceptibility to fluoroquinolones), a third-generation cephalosporin, and any

other antibiotic currently used for treatment.

Prevention

Vaccines are available for typhoid fever and bubonic plague; however, neither is routinely recommended in the

United States. An oral, multiple-dose vaccine prepared against S. serotype Typhi strain Ty2la or a parenteral

single-dose vaccine containing Vi antigen is available for people traveling to an endemic area or for household

contacts of a documented S. serotype Typhi carrier.

An inactivated multiple-dose, whole-cell bacterial vaccine is available for bubonic plague for people traveling

to an endemic area. However, this vaccine does not provide protection against pneumonic plague. Individuals

exposed to pneumonic plague should be given chemoprophylaxis with doxycycline (adults) or trimethoprim/

sulfamethoxazole (children younger than 8 years of age)

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Enterobacteriaceae

Genera and species to be considered

Opportunistic Pathogens:

Citrobacter freundii

Citrobacter (diversus) koseri

Citrobacter braakii

Cronobacter sakazakii (previously Enterobacter sakazakii)

Edwardsiella tarda

Enterobacter aerogenes

Enterobacter cloacae

Enterobacter gergoviae

Enterobacter amnigenus

Enterobacter (cancerogenous) taylorae

Escherichia coli (including extraintestinal)

Ewingella americana

Hafnia alvei

Klebsiella pneumoniae

Klebsiella oxytoca

Morganella morganii subsp. morganii

Morganella psychrotolerans

Pantoea agglomerans (previously Enterobacter agglomerans)

Proteus mirabilis

Proteus vulgaris

Proteus penneri

Providencia alcalifaciens

Providencia heimbachae

Providencia rettgeri

Providencia stuartii

Serratia marcescens

Serratia liquefaciens group

Serratia odorifera

Pathogenic Organisms:

Primary Intestinal Pathogens

E. coli (diarrheagenic)

Plesiomonas shigelloides

Salmonella, all serotypes

Shigella dysenteriae (group A)

Shigella flexneri (group B)

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Shigella boydii (group C)

Shigella sonnei (group D)

Pathogenic Yersinia spp.

Yersinia pestis

Yersinia enterocolitica subsp. enterocolitica

Yersinia frederiksenii

Because of the large number and diversity of genera included in the Enterobacteriaceae, it is helpful to consider

the bacteria of this family as belonging to one of two major groups. The first group comprises species that

either commonly colonize the human gastrointestinal tract or are most notably associated with human

infections. Although many Enterobacteriaceae that cause human infections are part of our normal

gastrointestinal flora, there are exceptions, such as Yersinia pestis. The second group consists of genera capable

of colonizing humans but rarely associated with human infection and commonly recognized as environmental

inhabitants or colonizers of other animals. For this reason, the discovery of these species in clinical specimens

should alert laboratorians to possible identification errors; careful confirmation of both the laboratory results

and the clinical significance of such isolates is warranted.

General Characters

 Molecular analysis has not proven effective for definitively characterizing all the organisms and genera

included within the Enterobacteriaceae family. Therefore, species names and reclassification of organisms

continually evolve. In general, the Enterobacteriaceae consist of a diverse group of gram negative bacilli or

coccobacilli; they are non–spore forming, facultative anaerobes capable of fermenting glucose; they are oxidase

negative (except for Plesiomonas sp.); and, with rare exception (Photorhabdus and Xenorhabdus spp.), they

reduce nitrates to nitrites. Furthermore, except for Shigella dysenteriae type 1, all commonly isolated

Enterobacteriaceae are catalase positive.

Epidemiology

Enterobacteriaceae inhabit a wide variety of niches, including the human gastrointestinal tract, the

gastrointestinal tract of other animals, and various environmental sites. Some are agents of zoonoses, causing

infections in animal populations (Table 1). Just as the reservoirs for these organisms vary, so do their modes of

transmission to humans. For species capable of colonizing humans, infection may result when a patient’s own

bacterial strains (i.e., endogenous strains) establish infection in a normally sterile body site. These organisms

can also be passed from one patient to another. Such infections often depend on the debilitated state of a

hospitalized patient and are acquired during the patient’s hospitalization (nosocomial). However, this is not

always the case. For example, although E. coli is the most common cause of nosocomial infections, it is also the

leading cause of community-acquired urinary tract infections. Other species, such as Salmonella spp., Shigella

spp., and Yersinia enterocolitica, inhabit the bowel during infection and are acquired by ingestion of

contaminated food or water. This is also the mode of transmission for the various types of E. coli known to

cause gastrointestinal infections. In contrast, Yersinia pestis is unique among the Enterobacteriaceae that infect

humans. This is the only species transmitted from animals by an insect vector (i.e., flea bite).

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Table (1-1) Epidemiology of Clinically Relevant Enterobacteriaceae

Organism Habitat (Reservoir) Mode of Transmission

Varies with the type of infection. For

nongastrointestinal

infections, organisms may be endogenous or spread

person to person, especially in the hospital setting.

For gastrointestinal infections, the transmission mode

varies with the strain of E. coli (see Table 20-2); it

may involve fecal-oral spread between humans in

contaminated food or water or consumption of

undercooked beef or unpasteurized milk from

colonized cattle

Normal bowel flora of humans

and

other animals; may also inhabit

female genital tract

Escherichia coli

Person-to-person spread by fecal-oral route,

especially

in overcrowded areas, group settings (e.g., daycare)

and areas with poor sanitary conditions

Only found in humans at times of

infection; not part of normal

bowel flora

Shigella spp

Person-to-person spread by fecal-oral route by

ingestion of food or water contaminated with human

excreta

Only found in humans but not

part

of normal bowel flora

Salmonella serotype

Typhi

Salmonella serotypes

Paratyphi A, B, C

Ingestion of contaminated food products processed

from animals, frequently of poultry or dairy origin.

Direct person-to-person transmission by fecal-oral

route can occur in health care settings when

hand-washing guidelines are not followed

. Widely disseminated

in nature and

associated with various animals

Other Salmonella spp

Uncertain; probably by ingestion of contaminated

water

or close contact with carrier animal

Gastrointestinal tract of coldblooded

animals, such as reptiles

Edwardsiella tarda

From rodents to humans by the bite of flea vectors or

by ingestion of contaminated animal tissues; during

human epidemics of pneumonic (i.e., respiratory)

Carried by urban and domestic

rats

and wild rodents, such as the

Yersinia pestis

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Pathogenesis and spectrum of diseases:

 The clinically relevant members of the Enterobacteriaceae can be considered as two groups: the opportunistic

pathogens and the intestinal pathogens.

 Typhi and Shigella spp. are among the latter group and are causative agents of typhoid fever and dysentery,

respectively. Yersinia pestis is not an intestinal pathogen, but it is the causative agent of plague. The

identification of these

organisms in clinical material is serious and always significant. These organisms, in addition to others, produce

various potent virulence factors and can cause life threatening infections (Table 2). The opportunistic pathogens

most commonly include Citrobacter spp., Enterobacter spp., Klebsiella spp., Proteus spp., Serratia spp., and a

variety of other organisms. Although considered opportunistic pathogens, these organisms produce significant

virulence factors, such as endotoxins capable of mediating fatal infections.

However, because they generally do not initiate disease in healthy, uncompromised human hosts, they are

considered opportunistic.

disease, the organism can be spread directly from

human to human by inhalation of contaminated

airborne droplets; rarely transmitted by handling or

inhalation of infected animal tissues or fluids

ground squirrel, rock squirrel, and

prairie dog

Consumption of incompletely cooked food products

(especially pork), dairy products such as milk, and,

less commonly, by ingestion of contaminated water

or by contact with infected animals

 Dogs, cats, rodents,

rabbits, pigs,

sheep, and cattle; not part of

normal human microbiota

Yersinia

enterocolitica

Ingestion of organism during contact with infected

animal or from contaminated food or water

Rodents, rabbits, deer, and birds;

not

part of normal human microbiota

Yersinia

pseudotuberculosis

Endogenous or person-to-person spread, especially in

hospitalized patients

Normal human gastrointestinal

microbiota

Citrobacter spp.,

Enterobacter spp.,

Klebsiella spp.,

Morganella spp.,

Proteus

spp., Providencia

spp., and Serratia spp

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Although E. coli is a normal bowel inhabitant, its pathogenic classification is somewhere between that of the

overt pathogens and the opportunistic organisms. Diuretic strains of this species, such as enterotoxigenic

E. coli (ETEC), enteroinvasive E. coli (EIEC), and enteroaggregative E. coli (EAEC), express potent toxins and

cause serious gastrointestinal infections. Additionally, in the case of enterohemorrhagic E. coli (EHEC) also

referred to as verocytotoxin producing E. coli (VTEC) or Shiga-like toxin producing E. coli (STEC), the

organism may produce life-threatening systemic illness. Furthermore, as the leading cause of

Enterobacteriaceae nosocomial infection, E. coli is likely to have greater virulence capabilities than the other

species categorized as “opportunistic” Enterobacteriaceae.

Table (1-2 )Pathogenesis and Spectrum of Disease for Clinically Relevant Enterobacteriaceae

Organism Virulence Factors Spectrum of Disease and Infections

Escherichia coli

(as a cause of

extraintestinal

infections)

Several, including endotoxin,

capsule

production pili that mediate

attachment to host cells

Urinary tract infections, bacteremia, neonatal

meningitis, and

nosocomial infections of other various body sites.

Most common

cause of gram-negative nosocomial infections.

Enterotoxigenic

E. coli

(ETEC)

Pili that permit gastrointestinal

colonization. Heat-labile (LT)

and

heat-stable (ST) enterotoxins

that

mediate secretion of water and

electrolytes into the bowel

lumen

Traveler’s and childhood diarrhea, characterized by

profuse, watery

stools. Transmitted by contaminated food and water.

Enteroinvasive

E. coli (EIEC)

Virulence factors uncertain, but

organism invades enterocytes

lining

the large intestine in a manner

nearly

identical to Shigella

Dysentery (i.e., necrosis, ulceration, and

inflammation of the large

bowel); usually seen in young children living in

areas of poor

sanitation.

Enteropathogenic

E. coli (EPEC)

Bundle-forming pilus, intimin,

and other

factors that mediate organism

attachment to mucosal cells of

the

small bowel, resulting in

changes in

cell surface (i.e., loss of

Diarrhea in infants in developing, low-income

nations; can cause a

chronic diarrhea.

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microvilli

Enterohemorrhagic

E. coli (EHEC,

VTEC, or STEC)

Toxin similar to Shiga toxin

produced by

Shigella dysenteriae. Most

frequently

associated with certain

serotypes,

such as E. coli O157:H7

Inflammation and bleeding of the mucosa of the

large intestine (i.e.,

hemorrhagic colitis); can also lead to hemolyticuremic syndrome,

resulting from toxin-mediated damage to kidneys.

Transmitted by

ingestion of undercooked ground beef or raw milk.

Enteroaggregative

E. coli (EAEC)

Probably involves binding by

pili, ST-like,

and hemolysin-like toxins;

actual

pathogenic mechanism is

unknown

Watery diarrhea that in some cases can be prolonged.

Mode of

transmission is not well understoo

Shigella spp. Several factors involved to

mediate

adherence and invasion of

mucosal

cells, escape from phagocytic

vesicles, intercellular spread,

and

inflammation. Shiga toxin role

in

disease is uncertain, but it does

have

various effects on host cells.

Dysentery defined as acute inflammatory colitis and

bloody diarrhea

characterized by cramps, tenesmus, and bloody,

mucoid stools.

Infections with S. sonnei may produce only watery

diarrhea

Salmonella serotypes Several factors help protect

organisms

from stomach acids, promote

attachment and phagocytosis

by

intestinal mucosal cells, allow

survival

in and destruction of

phagocytes, and

facilitate dissemination to other

tissues.

Three general categories of infection are seen:

• Gastroenteritis and diarrhea caused by a wide

variety of serotypes

that produce infections limited to the mucosa and

submucosa of the

gastrointestinal tract. S. serotype Typhimurium and

S. serotype

Enteritidis are the serotypes most commonly

associated with

Salmonella gastroenteritis in the United States.

• Bacteremia and extraintestinal infections occur by

spread from

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the gastrointestinal tract. These infections usually

involve

S. Choleraesuis or S. dublin, although any serotype

may cause these

infections.

• Enteric fever (typhoid fever, or typhoid) is

characterized by prolonged

fever and multisystem involvement, including blood,

lymph nodes,

liver, and spleen. This life-threatening infection is

most frequently

caused by S. serotype Typhi; more rarely, S.

serotypes Paratyphi A, B

or C.

Yersinia pestis Multiple factors play a role in

the

pathogenesis of this highly

virulent

organism. These include the

ability to

adapt for intracellular survival

and

production of an

antiphagocytic

capsule, exotoxins, endotoxins,

coagulase, and fibrinolysin

Two major forms of infection are bubonic plague

and pneumonic

plague. Bubonic plague is characterized by high

fever and painful

inflammatory swelling of axilla and groin lymph

nodes (i.e., the

characteristic buboes); infection rapidly progresses to

fulminant

bacteremia that is frequently fatal if untreated.

Pneumonic plague

involves the lungs and is characterized by malaise

and pulmonary

signs; the respiratory infection can occur as a

consequence of

bacteremic spread associated with bubonic plague or

can be

acquired by the airborne route during close contact

with other

pneumonic plague victims; this form of plague is

also rapidly fatal.

Yersinia

enterocolitica

subsp.

enterocolitica

Various factors encoded on a

virulence

plasmid allow the organism to

attach

to and invade the intestinal

Enterocolitis characterized by fever, diarrhea, and

abdominal pain; also

can cause acute mesenteric lymphadenitis, which

may present

clinically as appendicitis (i.e., pseudoappendicular

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mucosa

and spread to lymphatic tissue.

syndrome).

Bacteremia can occur with this organism but is

uncommon

Yersinia

Pseudotuber

culosis

Similar to those of Y.

enterocolitica

Causes infections similar to those described for Y.

enterocolitica but is

much less common

Citrobacter spp.,

Enterobacter spp.,

Klebsiella spp.,

Morganella spp.,

Proteus spp.,

Providencia spp.,

and Serratia spp.

Several factors, including

endotoxins,

capsules, adhesion proteins,

and

resistance to multiple

antimicrobial

agents

Wide variety of nosocomial infections of the

respiratory tract, urinary

tract, blood, and several other normally sterile sites;

most frequently

infect hospitalized and seriously debilitated patients

Specific organisms:

Opportunistic human pathogens

Citrobacter spp. (C. freundii, C. koseri, C. braakii) Citrobacter organisms are inhabitants of the intestinal

tract.The most common clinical manifestation in patients as a result of infection occurs in the urinary tract.

However,additional infections, including septicemias, meningitis, brain abscesses, and neurologic

complications, have been person to person. ( Table 3) provides an outline of the biochemical differentiation of

the most common clinically isolated Citrobacter species. C. freundii may harbor inducible AmpC genes that

encode resistance to ampicillin and first-generation cephalosporins.

Table(3) Biochemical Differentiation of Citrobacter Species

Species Indole ODC Malonate ACID

FERMENTATIO

N

Adonitol

Dulcitol Melibiose Sucrose

C. braakii V pos Neg neg v v neg

C. freundii V neg Neg neg neg pos