Chapter 16 - Enterobacteriaceae

MLAB 2434 – Clinical MicrobiologyCecile Sanders & Keri Brophy-Martinez

Chapter 16 - Enterics

Family Enterobacteriaceae often referred to as “enterics”

Four major features:All ferment glucose (dextrose)All reduce nitrates to nitritesAll are oxidase negativeAll except Klebsiella, Shigella and

Yersinia are motile

Microscopic and Colony Morphology

Gram negative bacilli or coccobacilli

Non-spore forming Colony morphology on BAP or CA

of little value, as they look the same, except for Klebsiella

Selective and differential media are used for initial colony evaluation (ex. MacConkey, HE, XLD agars)

Classification of Enterics

Due to the very large number of organisms in the Family Enterobacteriaceae (see Table 16-11), species are grouped into Tribes, which have similar characteristics (Table 16-1, page 466)

Within each Tribe, species are further subgrouped under genera

Virulence and Antigenic Factors of Enterics Ability to colonize, adhere, produce

various toxins and invade tissues Some possess plasmids that may

mediate resistance to antibiotics Many enterics possess antigens that

can be used to identify groups O antigen – somatic, heat-stable

antigen located in the cell wall H antigen – flagellar, heat labile antigen K antigen – capsular, heat-labile

antigen

Clinical Significance of Enterics Enterics are ubiquitous in nature Except for few, most are present

in the intestinal tract of animals and humans as commensal flora; therefore, they are sometimes call “fecal coliforms”

Some live in water, soil and sewage

Clinical Significance of Enterics (cont’d) Based on clinical infections

produced, enterics are divided into two categories:Opportunistic pathogens –

normally part of the usual intestinal flora that may produce infection outside the intestine

Primary intestinal pathogens – Salmonella, Shigella, and Yersinia sp.

Escherichia coli

Most significant species in the genus

Important potential pathogen in humans

Common isolate from colon flora

Escherichia coli (cont’d)

CharacteristicsDry, pink (lactose positive) colony

with surrounding pink area on MacConkey

Escherichia coli (cont’d)

Ferments glucose, lactose, trehalose, & xylose

Positive indole and methyl red tests

Does NOT produce H2S or phenylalanine deaminase

Simmons citrate negativeUsually motileVoges-Proskauer test negative

Escherichia coli (cont’d)

InfectionsWide range including meningitis,

gastrointestinal, urinary tract, wound, and bacteremia

Gastrointestinal Infections • Enteropathogenic (EPEC) – primarily

in infants and children; outbreaks in hospital nurseries and day care centers; stool has mucous but not blood; identified by serotyping

Escherichia coli (cont’d)

• Enterotoxigenic (ETEC) – “traveler’s diarrhea”; watery diarrhea without blood; self-limiting; usually not identified, other than patient history and lactose-positive organisms cultured on differential media

• Enteroinvasive (EIEC) – produce dysentery with bowel penetration, invasion and destruction of intestinal mucosa; watery diarrhea with blood; do NOT ferment lactose; identified via DNA probes

Escherichia coli (cont’d)

• Enterohemorrhagic (EHEC serotype 0157:H7) – associated with hemorrhagic diarrhea and hemolytic-uremic syndrome (HUS), which includes low platelet count, hemolytic anemia, and kidney failure; potentially fatal, especially in young children; undercooked hamburger, unpasteurized milk and apple cider have spread the infection; does NOT ferment sucrose; identified by serotyping

Escherichia coli (cont’d)

• Enteroaggregative (EaggEC) – cause diarrhea by adhering to the mucosal surface of the intestine; watery diarrhea; symptoms may persist for over two weeks

Urinary Tract Infections• E. coli is most common cause of UTI

and kidney infection in humans• Usually originate in the large

instestine• Able to adhere to epithelial cells in the

urinary tract

Escherichia coli (cont’d)

Septicemia & Meningitis• E. coli is one of the most common causes of

septicemia and meningitis among neonates; acquired in the birth canal before or during delivery

• E. coli also causes bacteremia in adults, primarily from a genitourinary tract infection or a gastrointestinal source

Escherichia hermannii – yellow pigmented; isolated from CSF, wounds and blood

Escherichia vulneris - wounds

Klebsiella, Enterobacter, Serratia & Hafnia sp. Usually found in intestinal tract Wide variety of infections, primarily

pneumonia, wound, and UTI General characteristics:

Some species are non-motile Simmons citrate positive H2S negative Phenylalanine deaminase negative Some weakly urease positive MR negative; VP positive

Klebsiella species

Usually found in GI tract Four major species K. pneumoniae is mostly commonly

isolated species Possesses a polysaccharide capsule,

which protects against phagocytosis and antibiotics AND makes the colonies moist and mucoid

Has a distinctive “yeasty” odor Frequent cause of nosocomial

pneumonia

Klebsiella species (cont’d)

Significant biochemical reactions• Lactose positive• Most are urease positive• Non-motile

Enterobacter species

Comprised of 12 species; E. cloacae and E. aerogenes are most common

Isolated from wounds, urine, blood and CSF

Major characteristicsColonies resemble KlebsiellaMotileMR negative; VP positive

Enterobacter species(cont’d)

Serratia species

Seven species, but S. marcescens is the only one clinically important

Frequently found in nosocomial infections of urinary or respiratory tracts

Implicated in bacteremic outbreaks in nurseries, cardiac surgery, and burn units

Fairly resistant to antibiotics

Serratia species (cont’d)

Major characteristicsFerments lactose slowlyProduce characteristic pink

pigment, especially when cultures are left at room temperature

S. marscens on nutrient agar →

Hafnia species

Hafnia alvei is only species Has been isolated from many

anatomical sites in humans and the environment

Occasionally isolated from stools Delayed citrate reaction is major

characteristic

Proteus, Morganella & Providencia species All are normal intestinal flora Opportunistic pathogens Deaminate phenylalanine All are lactose negative

Proteus species

P. mirabilis and P. vulgaris are widely recognized human pathogens

Isolated from urine, wounds, and ear and bacteremic infections

Both produce swarming colonies on non-selective media and have a distinctive “burned chocolate” odor

Both are strongly urease positive Both are phenylalanine deaminase

positive

Proteus species (cont’d)

A exhibits characteristic “swarming” B shows urease positive on right

Morganella species

Morganella morganii is only species

Documented cause of UTI Isolated from other anatomical

sites Urease positive Phenylalanine deaminase

positive

Providencia species

Providencia rettgeri is pathogen of urinary tract and has caused nosocomial outbreaks

Providenicia stuartii can cause nosocomial outbreaks in burn units and has been isolated from urine

Both are phenylalanine deaminase positive

Citrobacter species

Citrobacter freundii associated with nosocomial infections (UTI, pneumonias, and intraabdominal abscesses)

Ferments lactose and hydrolyzes urea slowly

Resembles Salmonella sp.

Salmonella

Produce significant infections in humans and certain animals

On differential selective agar, produces clear, colorless, non-lactose fermenting colonies with black centers (if media contains indicator for hydrogen sulfide)

Salmonella (cont’d)

Salmonella on MacConkey

Salmonella (cont’d)

Lactose negative Negative for indole, VP,

phenylalanine deaminase, and urease

Most produce H2S Do not grow in potassium cyanide Large and complex group of

organisms; grouped by O, H, and Vi (for virulence) antigens

Salmonella (cont’d)

Clinical Infections Acute gastroenteritis or food poisoning

• Source = handling pets, insufficiently cooked eggs and chicken, and contaminated cooking utensils

• Occurs 8 to 36 hours after ingestion• Requires a high microbial load for infection• Self-limiting in health individuals (antibiotics

and antidiarrheal agents may prolong symptoms)

Salmonella (cont’d)

Typhoid and Other Enteric Fevers• Prolonged fever• Bacteremia• Involvement of the RE system,

particularly liver, spleen, intestines, and mesentery

• Dissemination to multiple organs• Occurs more often in tropical and

subtropical countries

Salmonella (cont’d)

Salmonella BacteremiaCarrier State

• Organisms shed in feces• Gallbladder is the site of organisms

(removal of gallbladder may be the only solution to carrier state)

Shigella species

Closely related to the Escherichia All species cause bacillary

dysentery S. dysenteriae (Group A) S. flexneri (Group B) S. boydii (Group C) S. sonnei (Group D)

Shigella (cont’d)

Characteristics Non-motile Do not produce gas from glucose Do not hydrolyze urea Do not produce H2S on TSI Lysine decarboxylase negative ONPG positive (delayed lactose +) Fragile organisms Possess O and some have K antigens

Shigella (cont’d)

Clinical Infections Cause dysentery (bloody stools,

mucous, and numerous WBC) S. sonnei is most common, followed by

S. flexneri (“gay bowel syndrome”) Humans are only known reservoir Oral-fecal transmission Fewer than 200 bacilli are needed for

infection in health individuals

Shigella (cont’d)

Yersinia species

Consists of 11 named species Yersinia pestis

Causes plague, which is a disease primarily of rodents; transmitted by fleas

Two forms of plague, bubonic and pneumonic

Gram-negative, short, plump bacillus, exhibiting “safety-pin” or “bipolar” staining

Yersinia species

Yersinia enterocolitica Most common form of Yersinia Found worldwide Found in pigs, cats and dogs Human also infected by ingestion of

contaminated food or water Some infections result from eating

contaminated market meat and vacuum-packed beef

Is able to survive refrigerator temperatures (can use “cold enrichment” to isolate)

Mainly causes acute gastroenteritis with fever

Yersinia species

Yersinia pseudotuberculosisPathogen of rodents, particularly

guinea pigsSepticemia with mesenteric

lymphadenitis, similar to appendicitis

Motile at 18 to 22 degrees C

Laboratory Diagnosis of Enterics Collection and Handling

If not processed quickly, should be collected and transported in Cary-Blair, Amies, or Stuart media

Isolation and IdentificationSite of origin must be consideredEnterics from sterile body sites

are highly significantRoutinely cultured from stool

Laboratory Diagnosis of Enterics (cont’d) Media for Isolation and

Identification of EntericsMost labs use BAP, CA and a

selective/differential medium such as MacConkey

On MacConkey, lactose positive are pink; lactose negative are clear and colorless

Laboratory Diagnosis of Enterics (cont’d)

For stools, highly selective media, such as Hektoen Enteric (HE), XLD, or SS is used along with MacConkey agar

IdentificationMost labs use a miniaturized or

automated commercial identification system, rather than multiple tubes inoculated manually

Laboratory Diagnosis of Enterics (cont’d) Identification (cont’d)

All enterics are• Oxidase negative• Ferment glucose• Reduce nitrates to nitrites

Laboratory Diagnosis of Enterics (cont’d) Common Biochemical Tests

Lactose fermentation and utilization of carbohydrates

Triple Sugar Iron (TSI)ONPGGlucose metabolism

• Methyl red• Voges-Proskauer

Laboratory Diagnosis of Enterics (cont’d) Common Biochemical Tests

(cont’d)Miscellaneous Reactions

• Indole• Citrate utilization• Urease production• Motility• Phenylalanine deaminase• Decarboxylase tests

Screening Stools for Pathogens Because stools have numerous

microbial flora, efficient screening methods must be used to recover any pathogens

Enteric pathogens include Salmonella, Shigella, Aeromonas, Campylobacter, Yersinia, Vibrio, and E. coli 0157:H7

Screening Stools for Pathogens (cont’d) Most labs screen for Salmonella,

Shigella, and Campylobacter; many screen for E. coli 0157:H7

Fecal pathogens are generally lactose-negative (although Proteus, Providencia, Serratia, Citrobacter and Pseudomonas are also lactose-negative)