Microbiology Quiz: Enterobacteriaceae

Chapter 19: Enterobacteriaceae

• Enterobacteriaceae are a family of gram-negative, facultative anaerobes, typically coccobacilli or straight rods
• They ferment glucose and reduce nitrate to nitrites
• They are oxidase negative, except for Plesiomonas

General Characteristics

• Most ferment glucose
• All reduce nitrate to nitrites
• All are oxidase negative, except for Plesiomonas
• Most are motile at body temperature, except for Klebsiella, Shigella, and Yersinia

Colony Morphology

• Gram-negative coccobacilli or straight rods
• Not useful in identification except to rule out other organisms
• Facultatively anaerobic growth occurs throughout the thioglycolate broth
• Large moist, gray colonies
• Some colonies may appear mucoid, e.g., Klebsiella

Growth on Media

• MacConkey (MAC) agar: Selective and differential; bile salts and crystal violet inhibit gram-positive bacteria; lactose fermentation is differential

• Eosin-methylene blue (EMB) agar: Selective and differential; methylene blue inhibits gram-positive bacteria; lactose and sucrose fermentation are differential

• Hektoen enteric (HE) agar: Selective and differential; bile salts inhibit gram-positive bacteria, some gram-negative; lactose and sucrose fermentation are differential; some pathogens show different colors (e.g., green to blue-green) with hydrogen sulfide (H₂S) production

• Xylose lysine deoxycholate (XLD) agar: Selective (less so than MAC and HE) and differential; sodium desoxycholate inhibits gram-positive bacteria, some gram-negative; three carbohydrates (sucrose, lactose, and xylose) and a phenol red indicator are present; lysine decarboxylation and hydrogen sulfide (H₂S) production are used to differentiate

• XLD agar (Cont.):

  • Yellow colonies: fermenters or non-lysine-decarboxylase-producing organisms like Escherichia coli, Citrobacter
  • Colorless or red colonies: Shigella
  • Red with black centers: initially yellow, then revert to red due to lysine decarboxylation (alkaline pH); characteristic of Salmonella

Classification of Selected Species

• Includes various genera and species within the Enterobacteriaceae family, such as Escherichia coli, Salmonella, Shigella, Edwardsiella, Klebsiella, Enterobacter, Serratia, etc. (Full taxonomy table presented)

Biochemical Characteristics of Tribes of Enterobacteriaceae

• A table is provided (Table 19-2) showcasing various tests and substrates, and the relative positive/negative results for several species and tribes.

Virulence and Antigenic Factors

• Adherence factors that prevent removal from tissues
• Toxins (e.g., food poisoning)
• Invasive enzymes involved in spreading
• Antibiotic resistance (Extended-spectrum β-Lactamase [ESBL])
• Serology:
  • O antigen (somatic antigen), heat stable, located in the cell wall of LPS
  • H antigen (flagellar antigen), heat stable
  • K antigen (capsular antigen), heat labile
• Specific examples of antigens include K1 antigen of E. coli, Vi antigen of Salmonella typhi

Clinical Significance

• Two major types of pathogens:
  • Opportunistic pathogens (normal flora):
    • Infections in non-normal sites (e.g., septicemia, wounds, UTI, meningitis). Example: E.coli
  • Primary pathogens:
    • Commonly isolated from contaminated food and water. Examples: Salmonella, Shigella, Yersinia

Bacterial Species and Infections

• A table (Table 19-3) listing bacterial species and specific infections

E. coli

• Dry pink colonies on MAC plates
• β-hemolysis on blood agar (BAP)
• Motile with sex pili, fimbriae
• Possess O, H, and K antigens
• Biochemical reactions: glucose, lactose, trehalose, and xylose fermentation; indole production; methyl red (MR) positive; does not produce H2S, DNase, urease, or PAD; citrate negative

Clinical Types of E. coli

• Includes various types of pathogenic E. coli, such as uropathogenic E. coli (UPEC), enterohemorrhagic E. coli (EHEC), enteroinvasiive E. coli (EIEC), enteroaggregative E. coli (EAEC) and so forth. These are further defined by disease symptoms and other factors.

Extraintestinal Infections of E. coli

• Septicemia and meningitis most common in neonates and young children.
• Infection commonly occurs just before or during birth.

Other Escherichia Species

• Includes species like E. hermannii and E. vulneris, outlining their characteristics (color, isolation sites, and significance)

Klebsiella, Enterobacter, Serratia, Pantoea, Cronobacter, and Hafnia

• Tribe Klebsielleae pathogens. Characteristics: Simmons citrate positive, potassium cyanide broth negative, H₂S and phenylalanine deaminase negative, usually urease negative, indole negative, VP positive.

Klebsiella Species

• K. pneumoniae is the most common isolate, particularly from lower respiratory tract infections.
• Virulence factor is a polysaccharide capsule that prevents phagocytosis.

Differentiating Klebsiella Species

• Table (Table 19-5) to differentiate among different Klebsiella species

Raoutella Species

• R. ornithinolytica and R. planticola are indole and ornithine decarboxylase positive and found in urine, respiratory tracts, and blood.

Enterobacter Species

• E. cloacae and E. aerogenes are the most common species.
• E. gergoviae is mainly found in respiratory samples, and rarely in blood.
• E. hormaechei is another Enterobacter species.
• Characteristics: citrate positive, MR negative, VP positive, usually produce ornithine decarboxylase, found in wound sites, urine, and blood

Diagnostic Features of Select Enterobacter Species

• Table (Table 19-6) describing tests for distinguishing different Enterobacter species

Pantoea and Cronobacter Species

• Pantoea agglomerans, a plant pathogen, is similar to Klebsiella pneumoniae.
• Cronobacter sakazakii causes neonatal meningitis and bacteremia through a powdered infant formula route.

Serratia Species

• All Serratia species are positive for ONPG (slow lactose), except for S. fonticola.
• Many Serratia spp. generate a red pigment (prodigiosin) at room temperature and are resistant to antimicrobials.
• Species like S. liquefaciens, S. rubidaea, and S. odorifera have been described.

Hafnia Species

• H. alvei and its biotype are linked to gastroenteritis and grow in beer wort.

Proteus Species

• P. mirabilis and P. vulgaris are separated from other Proteus spp. by positive phenylalanine deaminase.
• They are lactose negative, generally swarm on lab media, have a burnt chocolate odor, and produce H2S.
• P. mirabilis is indole negative and ornithine decarboxylase positive.
• P. vulgaris is indole positive and ornithine decarboxylase negative with sucrose fermentation.

Morganella Species

• M. morganii is prominent in UTI infections and can occasionally be a diarrhea agent.

Providencia Species

• P. rettgeri, P. aalcalifaciens, and P. stuartii are species found in urinary tract infections and occasionally in hospital outbreaks.

Differentiating Characteristics of Proteus, Providencia, and Morganella

• A table (Table 19-7) to differentiate among different Proteus, Providencia, and Morganella species.

Edwardsiella

• E. tarda is the only pathogenic species in the genus. General characteristics involve: urea negative, lysine decarboxylase positive, H2S positive, indole positive, and no citrate growth. Pathogenic via bacteremia and wound infections

Erwinia and Pectobacterium

• These are plant pathogens

Citrobacter Species

• Includes C. freundii (nosocomial UTIs, pneumonias, endocarditis in drug abusers, intra-abdominal abscesses), C. koseri, and C. braakii (nursery outbreaks of neonatal meningitis and brain abscesses).
• Characteristic features for identifying Citrobacter include weak urease activity, lactose fermentation, and growth on Simmons citrate. Citrobacter is similar to Salmonella, in that some members hydrolyze urea but do not decarboxylate lysine, whereas most Salmonella do not hydrolyze urea, and do decarboxylate lysine.

Laboratory Diagnosis of Enterobacteriaceae

• Specimen collection and transport
• Direct microscopic examination: Not very helpful to differentiate
• Culture: generally grows quickly
• Screening stool cultures for pathogens: to separate from normal flora

Lysine Iron Agar (LIA) and TSI Screening

• A table showcasing differentiating features of relevant organisms in the Enterobacteriaceae family via LIA and TSI assays.

Hektoen Enteric and Xylose Lysine Deoxycholate (XLD) Agar

• Images showcasing the appearance of various relevant organisms on these media

CHROMagar™

• A growth medium to differentiate enteric bacteria through color changes

Identification

• Gram-negative Enterobacteriaceae are oxidase-negative, except for Plesiomonas.
• Use young colonies from sheep blood agar
• Must ferment glucose, reduce nitrate to nitrite, except for Photorhabdus and Xenorhabdus.

Serologic Grouping

• Salmonella has 60 types of O antigens (95% are serogroups A through E1) identified via direct or latex agglutination.
• Shigella grouping includes A through D serogroups using serological methods.

Clinical Infections

• Includes acute gastroenteritis, carrier states, non-typhoidal bacteremia, typhoid fever. Typhoid fever can involve ingestion, invasion of the small intestine, the lymphatic system, and seed bloodstream. Non-typhoidal bacteremia includes fever with intermittent bacteremia, in two groups: young children with gastroenteritis and brief episodes of bacteremia, and adults with transient bacteremia often during gastroenteritis or symptoms of septicemia.

Characteristics of Shigella Species

• Nonmotile
• Do not commonly produce gas from glucose, except for some S. flexneri
• Do not produce urease
• They are not producing H₂S in TSI
• Do not decarboxylate lysine
• Cannot use acetate or mucate as carbohydrate sources
• S. sonnei is positive for ONPG and ornithine decarboxylase test
• S. flexneri is negative.

Clinical Infection of Shigella

• Clinical symptoms, low inoculum causing disease, spread via fecal-oral route.
• Shigella dysenteriae is the most intense form of the infection.

Shigella Species (Cont.)

• Classifies species based on disease severity, commonly in developing countries, and associated with high-risk situations like men who have sex with men.

Yersinia Species

• Plague (bubonic, septicemic, and pneumonic) is transmitted through bites from infected fleas (gram negative coccobacillus). Preferential growth is between 25-30C
• Y. enterocolitica is contracted through contact with swine, cats, and dogs. Symptoms include acute gastroenteritis, arthritis, erythema nodosum, and is particularly common in children aged 1-5.
• Y. pseudotuberculosis is found in rodents and birds and causes caseous swellings called pseudotubercles in mesenteric lymph nodes via fecal material transmission.

Differentiating Yersinia Species

• Cefsulodin, irgasan, novobiocin (CIN) agar and Yersinia-selective agar (YSA) are used to differentiate Y. pestis, Y. enterocolitica, and Y. pseudotuberculosis based on their motility and ornithine decarboxylase reactions.

New Genera of Enterobacteriaceae

• Descriptions of various newly classified genera.

Laboratory Diagnosis of Enterobacteriaceae

• General method for specimen collection and transport, isolation and identification, and stool culture for pathogens.

Lysine Iron Agar, and TSI Screening

• Table (19-11) describes how to use LIA and TSI agar screening tests to help in the identification of Enterobacteriaceae species.

Hektoen Enteric and Xylose Lysine Deoxycholate (XLD) Agar

• Images of Hektoen Enteric and XLD agar plates are referenced, showing the appearance of various relevant organisms on both media.

CHROMagar™

• A growth medium for differentiating enteric bacteria via color changes

Identification (Cont.)

• To identify Enterobacteriaceae organisms: they are Gram-negative, oxidase-negative (with some exceptions)
• Glucose fermentation, nitrate reduction to nitrite.

Schematic Diagram for Enterics

• A large diagram provides a flowchart to help in the identification of enteric bacteria based on a combination of tests such as lactose or non-lactose fermentation, oxidase, and motility.

Differentiating Characteristics of Enterobacteriaceae, cont.

• Tables (19-12 and 19-13) present detailed biochemical reactions for various species to help differentiate them.

Test your knowledge on Enterobacteriaceae, focusing on Shigella, Plesiomonas shigelloides, and Salmonella. This quiz will challenge your understanding of serogroups and unique properties of these bacteria. Perfect for microbiology students and enthusiasts.