The Enterobacteriaceae (Enterobacterales)

Introduction to Enterobacteriaceae

  • Enteric Gram-negative bacilli (GNB) are commonly referred to as Enterobacteriaceae.

  • Current classification separates them into seven different families, leading to the discussion of the order Enterobacterales instead.

  • The order was previously called Enterobacteriales, but this name was not validly published.

  • The accepted name is Enterobacterales (with no 'i').

General Characteristics of Enterobacteriaceae

  • Commonly referred to as GNRs (Gram-negative rods).

  • Sometimes known as gut bugs.

  • Most species are normal flora of the gastrointestinal (GI) tract.

    • Notable exceptions: Salmonella, Shigella, and Yersinia, which are pathogenic.

  • Clinical significance arises during specific diagnostic procedures (e.g., stool cultures).

  • Common clinical specimens include:

    • Stool: look only for true pathogens.

    • Wound, urine, blood, sputum: any Enterobacteriaceae may be a culprit.

  • Environmental sources include soil, water, and sewage (feces).

  • Metabolic characteristics:

    • Facultative anaerobes (able to grow in the presence or absence of oxygen).

    • Ferment glucose.

    • Oxidase-negative (with an exception for Plesiomonas, which is oxidase-positive).

    • Nitrate-positive (reduce nitrate to nitrite).

Members of Enterobacteriaceae

Key Genera and Species Included

  • Escherichia coli

  • Cronobacter sakazakii

  • Shigella dysenteriae

  • Shigella flexneri

  • Shigella boydii

  • Pantoea agglomerans

  • Shigella sonnei

  • Edwardsiella tarda

  • Proteus mirabilis

  • Proteus vulgaris

  • Salmonella enterica (including serotype S. typhi)

  • Providencia bongori

  • Providencia rettgeri

  • Citrobacter freundii

  • Klebsiella pneumoniae

  • Klebsiella oxytoca

  • Morganella morganii

  • Yersinia enterocolitica

  • Yersinia pestis

  • Raoultella spp.

  • Enterobacter cloacae

  • Serratia marcescens

Serological Characteristics of Enterobacteriaceae

  • Antigens:

    • O: Cell wall (Somatic) antigen, Stable.

    • K: Capsular (Envelope) antigen, Labile.

    • H: Flagellar antigen, Labile.

    • Vi: Capsular antigen specific to Salmonella typhi, Labile.

Escherichia coli Overview

  • Habitat: Found in both human and animal intestinal tracts.

  • Normal Flora: Most strains are non-pathogenic; however, some can cause disease.

Diseases Associated with E. coli

  • Diarrhea/Gastroenteritis: Often termed “travelers' diarrhea.”

  • Hemorrhagic colitis: Associated with E. coli O157:H7, causing HUS (Hemolytic Uremic Syndrome).

  • Urinary Tract Infections (UTIs): E. coli is the #1 cause of UTIs.

  • Other conditions: appendicitis, meningitis in newborns, peritonitis, gall bladder infections, endocarditis, food poisoning.

  • E. coli often represents opportunistic infections manifesting in various clinical specimens.

Key Strain: E. coli O157:H7

  • Reservoir: Intestinal tract of cattle (non-pathogenic to cattle).

  • Transmission: Fecal-oral route via contaminated food or water.

  • Sources of outbreaks:

    • Raw or undercooked beef.

    • Contaminated baby spinach (from environments adjacent to cattle pastures).

    • Petting zoos, Taco Bell, Jack in the Box, unpasteurized milk, and contaminated drinking water.

  • Demographics at Higher Risk: Children and elderly individuals.

  • Pathology: Produces Shiga-like toxin (STEC), leading to:

    • Hemorrhagic colitis: inflammation of the colon with bleeding (6% of cases).

    • Symptoms: vomiting, cramps, diarrhea, low-grade fever, and in some cases, asymptomatic.

  • Complications:

    • Hemolytic Uremic Syndrome (HUS): Toxins attack kidneys leading to kidney failure and low platelet counts, along with anemia from broken RBCs.

  • Incubation Period: 3-4 days (with a range of 1-10 days). The infectious period lasts from 1 to 3 weeks.

  • Mortality Rate: Approximately 5%.

Treatment Considerations for STEC

  • Antibiotics may increase the risk of developing HUS; the benefits of antibiotic treatment are not clearly established.

  • It is critical to rule out STEC before administering antibiotics.

  • Shiga Toxins: Could be of two types (1 and 2); Shiga toxin 2 is generally more virulent.

  • Testing for both STEC O157 and non-0157 STEC should be part of the diagnostic workup.

  • Reports of STEC must be submitted to public health authorities within a day if detected by labs.

Other Virulent E. coli Types

Enterotoxigenic (ETEC)

  • Produces enterotoxins that mediate the secretion of water and electrolytes into the bowel, leading to profuse, watery diarrhea often referred to as “traveler’s diarrhea.”

Enteroinvasive (EIEC)

  • Invades large intestinal lining similar to Shigella, causing dysentery characterized by necrosis, ulceration, and bowel inflammation, associated mainly with children in areas suffering from poor sanitation.

Enteropathogenic (EPEC)

  • Attaches to small bowel mucosal cells leading to cell surface changes, such as loss of microvilli, resulting in diarrhea (sometimes chronic) seen in infants in developing countries.

Enteroaggregative (EAEC)

  • Pathogenic mechanism is not well understood; transmission is also obscure. Causes prolonged watery diarrhea.

Laboratory Identification of E. coli

  • Culture Media: E. coli can be differentiated based on growth on various selective media types:

    • EMB (Eosin Methylene Blue): lactose-positive, producing blue-black colonies.

    • MacConkey Agar (MAC): lactose-positive, yielding pink colonies.

    • Sorbitol MacConkey (sMAC): E. coli O157:H7 is sorbitol negative (clear);

    • SBA (Sheep Blood Agar): can exhibit beta-hemolytic activity.

    • HE (Hektoen Enteric): lactose-positive production.

Shigella Overview

  • Responsible for Shigellosis, also known as bacillary dysentery.

  • Infects the small intestine while damaging the large intestine, causing acute inflammatory colitis, characterized by:

    • Cramps, tenesmus, bloody or mucoid stools, and fever.

  • Demographics: Most severe in children and elderly individuals.

  • Transmission: Human (feces) directly to human (oral); very contagious with an infectious dose of fewer than 50 bacteria, showing high pathogenicity.

  • Shigella is not normal flora.

  • Incubation Period: 1-7 days.

  • Utilizes the O antigen for serotyping, as it lacks H antigen due to being non-motile.

  • S. dysenteriae is associated with the most severe dysentery and has the potential to invade the bloodstream, maintaining a mortality rate of up to 20% due to complications like Shiga toxin production.

  • Pathogenic process:

    • Proliferates within intestinal epithelial cells, avoiding host immune responses, which leads to inflammation and ulceration.

Shigella Serogroups and Antigens

  • Serogroups Based on O Antigens:

    • A: Shigella dysenteriae - attacks large intestines.

    • B: Shigella flexneri - mild diarrhea, common in the US; linked with post-infectious arthritis in HLA-B27 positive patients.

    • C: Shigella boydii - mild diarrhea.

    • D: Shigella sonnei - mild diarrhea, frequently presents as just watery stool and is the most prevalent cause of shigellosis in the US.

  • Detection: Groups are defined through bacterial agglutination tests. If untyped, boiling the suspension can help reveal O Ag.

Salmonella Overview

  • Comprises of two significant species: Salmonella enterica and Salmonella bongori, with over 2,400 serotypes.

  • Majority are pathogenic, notably contributing to moderate to severe gastroenteritis, typically through strains of Salmonella enterica (including S. typhi, S. paratyphi, S. typhimurium, S. enteritidis).

  • Reservoirs: Many animal hosts, particularly reptiles and poultry; however, S. typhi is solely found in humans.

  • Transmission: Oral-fecal from contaminated water or undercooked food (especially chicken).

Pathology and Epidemiology of Salmonella enterica

  • Incubation Period: 12-36 hours.

  • Common Symptoms: moderate fever, nausea, abdominal pain, cramps, diarrhea.

  • Mortality Rates:

    • General population: low.

    • Very young and elderly: higher.

  • Primary sources of infections: Poultry and related products (including eggs) and unpasteurized milk.

  • The bacterium works intracellularly, multiplying inside the intestinal epithelial cells.

S. typhi Pathology

  • Causes Typhoid Fever.

  • Incubation Period: Approximately 2 weeks.

  • Symptoms: high fever, headache, diarrhea.

  • Mortality Rates: Ranging from 1-2% in developed nations, higher (10-30%) in undeveloped regions.

  • Spread involves the bacterium multiplying within phagocytic cells and disseminating systemically, often causing complications like organ failure.

  • Historical reference to “Typhoid Mary”, who unknowingly infected many due to her role as a cook.

Growth on Laboratory Media

  • All Salmonella species are typically lactose-negative.

  • Media types for selective isolation:

    • MacConkey Agar (MAC): colorless colonies.

    • XLD (Xylose Lysine Deoxycholate): red or red-black centers.

    • Hektoen Enteric (HE): green or black differentiation.

Detecting Foodborne Illnesses

Culture-Dependent Approaches

  • Uses specimens from patients (e.g., stool, blood, urine).

  • High accuracy but requires specialized knowledge.

  • Can produce pure cultures for subtyping and susceptibility testing.

Culture-Independent Approaches

  • Offers faster results but with a potentially lower accuracy rate.

  • Allows simultaneous testing for various pathogens, including bacteria, viruses, and parasites.

Edwardsiella Overview

  • Edwardsiella tarda is the most common species.

  • Rare causes for human infection; sources include the environment, snakes, and turtles.

  • Associated conditions: diarrhea, septicemia, and wounds.

  • Laboratory differentiation: resembles Salmonella (H2S positive, lactose negative) but is indole positive and citrate negative.

Klebsiella Overview

  • Common infections include UTIs and pneumonia featuring currant jelly sputum due to encapsulation.

  • K. pneumoniae is the most prevalent isolate with notable mucoid colonies.

  • Increased risk in populations: diabetics, alcoholics.

  • K. oxytoca is indole positive, while K. pneumoniae is not.

  • K. granulomatis, the agent of granuloma inguinale (an STD), does not grow on lab media but can be identified under specific microscopy techniques.

Enterobacter Overview

  • Comprises 12 species isolating from human/animal gut, soil, water, and dairy products.

  • Common infections include respiratory tract infections, wounds, and blood infections; E. cloacae is frequently isolated.

  • E. sakazakii (now Cronobacter sakazakii) is linked to food poisoning often from infant formula and associated with high infant mortality rates (up to 50%).

Serratia Overview

  • Recognized as an opportunistic pathogen typically affecting immunocompromised patients, particularly in NICU outbreaks.

  • S. marcescens produces a pigment depending on temperature and is a delayed lactose fermenter.

Proteus Overview

  • Pathogenic roles in UTIs and pneumonia, septicemia are attributed to P. vulgaris and P. mirabilis.

  • Produces H2S; swarming growth characteristic.

Providencia Overview

  • Infections include UTIs and pneumonia, mainly through species like P. rettgeri.

Morganella Overview

  • Containing only M. morganii, presents symptoms in UTIs, pneumonia, or septicemia.

Citrobacter Overview

  • Common species includes C. freundii, featuring infections in UTIs, pneumonia, and septicemia.

Yersinia Overview

  • Pathogenic species include:

    • Y. pestis: responsible for bubonic plague.

    • Y. enterocolitica: linked to enterocolitis and transmitted via contaminated food, water, or animal feces.

  • Notable for its motility at 25°C but is non-motile at 37°C, particularly Y. pestis.

Additional Pathology of Yersinia enterocolitica

  • Media for isolation includes CIN (Cefsulodin-Irgasan-Novobiocin) and it grows best at room temperature.

Bubonic Plague by Yersinia pestis

  • Historical concern associated with rapid human transmission via fleas. Characterized by high mortality rates without treatment, emphasizing the relevance of modern medicine in managing the risks associated with historically significant pathogens.

Laboratory Identification Tests

Carbohydrate Utilization Tests: KIA and TSI

  • KIA (Kligler Iron Agar) and TSI (Triple Sugar Iron): both test for carbohydrate fermentation, sulfur production, and gas production.

  • Results Interpretation: Reports of fermentation indicated by yellow coloration, black butt signifies H2S production, with various outputs denoting specific species characteristics.

Biochemical Identification Tests

  • Indole Test: Tests for tryptophanase presence; positive results indicated by color changes.

  • Methyl Red: Assesses acidity of the environment; indicates ability for mixed acid fermentation.

  • Voges-Proskauer (VP): Tests for acetoin production in fermentation.

  • Citrate Test: Indicates utilization of citrate as a carbon source, measured by pH changes in the media.

  • Motility Testing: Assesses the mobility of the organism, effectively distinguishing between motile and non-motile organisms.

Conclusion

  • The Enterobacteriaceae family showcases a diverse range of pathogenic organisms with significant clinical impact, necessitating understanding from metabolic, pathogenic, and laboratory identification perspectives for effective management in healthcare settings.