Which of the following is a true statement regarding Enterobacteriaceae?

All members are oxidase negative

Glucose is fermented by most members of the family

Motility in Enterobacteriaceae is via peritrichous flagella

A 23-year-old woman with a history of sickle cell disease presents with fever and severe bone pain localized to her left tibia. X-ray reveals a lytic lesion, and blood cultures reveal infection. A bone culture grows gram-negative rods. Which of the following best describes the infecting organism?

It is motile and does not ferment lactose, such as Salmonella.

It is a facultative intracellular parasite, such as Legionella.

It is a non-motile facultative anaerobe, such as Shigella.

It is comma-shaped and sensitive to acidic pH, such as Vibrio cholerae.

All are true about the bacteria shown in the figure except: (Recent NEET Pattern 2016-17)

Resistant to heat, alteration in pH and disinfectants

Commensal in upper respiratory tract of humans

Kovac's method results in formation of deep purple color

Nonhemolytic, gray translucent colonies

Enterobacteriaceae for FMGE: High-Yield Microbiology Lessons

Enterobacteriaceae: General Properties - Gut Gang Basics

Key features: Gram-negative rods, facultative anaerobes.
Biochemical tests: Ferment glucose, oxidase-negative (key differentiator!), catalase-positive (most species).
Metabolism: Reduce nitrates ($NO_3^-$) to nitrites ($NO_2^-$).
Common habitats: Intestinal tract of humans/animals, also soil and water.
Antigenic structure for serotyping:
- O antigen: Somatic, lipopolysaccharide (LPS).
- H antigen: Flagellar, protein.
- K antigen: Capsular, polysaccharide (Vi in Salmonella).

Enterobacteriaceae: Types, Background, Transmission

Classification by Lactose Fermentation (on MacConkey Agar):

⭐ All Enterobacteriaceae are oxidase-negative, a crucial diagnostic feature.

Escherichia coli - The Versatile Villain

Most common facultative anaerobe in the human colon.
Diarrheagenic E. coli (DEC) pathotypes:
- ETEC (Enterotoxigenic): Traveler's diarrhea; heat-labile (LT) & heat-stable (ST) toxins.
- EPEC (Enteropathogenic): Infantile diarrhea; attaching/effacing (A/E) lesions.
- EHEC/STEC (Enterohemorrhagic/Shiga toxin-producing): Hemorrhagic colitis, HUS; Shiga-like toxin; serotype O157:H7 is notorious.
- EIEC (Enteroinvasive): Invasive, dysentery-like illness.
- EAEC (Enteroaggregative): Persistent diarrhea, especially in children; biofilm formation.
Extra-intestinal Infections (ExPEC):
- UTI (Uropathogenic E. coli - UPEC): Most common cause.
- Neonatal meningitis (NMEC).
- Sepsis.
Key Virulence Factors: Adhesins (pili/fimbriae), toxins (e.g., LPS endotoxin), capsules (e.g., K1 in NMEC).

E. coli pathotypes and infection mechanisms

⭐ EHEC O157:H7 is associated with Hemolytic Uremic Syndrome (HUS) and characteristically does not ferment sorbitol (distinguished on sorbitol-MacConkey agar).

Salmonella & Shigella - Diarrhea Dynamos

Salmonella (motile, H₂S+) and Shigella (non-motile, H₂S-) are key Gram-negative bacilli causing distinct diarrheal illnesses. Differentiated by motility, H₂S production, and clinical syndromes.

Feature	Salmonella	Shigella
Motility	Motile	Non-motile
H₂S Production	Yes (most)	No
Lactose Ferm.	NLF (usually)	NLF
Clinical	Enteric Fever (S. Typhi/Paratyphi: Vi Ag, systemic) Gastroenteritis (Non-typhoidal: local)	Bacillary Dysentery (bloody, mucoid stools)
Infective Dose	Higher	10-100 organisms (very low)
Toxin/Virulence	Endotoxin, Vi Ag (Typhi)	Shiga toxin (S. dysenteriae), Invasiveness
Reservoir	Humans (Typhi), Animals (NTS e.g. poultry)	Humans

⭐ Shigella: non-motile, H₂S negative, very low infective dose (10-100). Salmonella: motile, H₂S positive (most).

Shigella pathogenesis in host cells

Other Enterobacteriaceae - The Notable Crew

Klebsiella: K. pneumoniae. Capsule (mucoid). LF. Pneumonia (currant jelly sputum), UTI. 📌 '5 A's': Aspiration, Alcoholics, Abscesses, di-A-betics, current-jelly sput-A.
Enterobacter: Motile, LF. UTI, nosocomial.
Serratia: S. marcescens. Motile. Red pigment (prodigiosin). Nosocomial.
Proteus: P. mirabilis/vulgaris. Highly motile (swarming). Urease +ve (struvite stones). NLF. Fishy odor. UTI.

⭐ P. mirabilis: swarming motility, urease +ve, forms struvite kidney stones.
Yersinia:
- Y. pestis: Plague. Safety pin. Fleas.
- Y. enterocolitica/pseudotuberculosis: Enterocolitis, mesenteric adenitis (mimics appendicitis). Cold enrichment. NLF.
Citrobacter: LF. UTI, neonatal sepsis/meningitis.

Lab Diagnosis & Treatment - ID & Attack Plan

Specimens: Stool, urine, blood, CSF, depending on infection site.
Culture:
- MacConkey Agar: Selective & differential (Lactose Fermenters - LF pink; Non-Lactose Fermenters - NLF colorless).
- EMB Agar: LF (E. coli - green metallic sheen), NLF.
- Hektoen Enteric (HE) / XLD Agar: For Salmonella ($H_2S$ positive → black colonies) & Shigella.
Biochem Tests:
- Oxidase Test: Negative for all Enterobacteriaceae.
- TSI Agar: Glucose, lactose, sucrose fermentation; gas & $H_2S$ production.
- 📌 IMViC (Indole, Methyl Red, Voges-Proskauer, Citrate): E.g., E.coli (++--), Klebsiella/Enterobacter (--++).
Serotyping: O, H, K antigens for epidemiological typing (e.g., Salmonella, E. coli).
Treatment: Guided by susceptibility. Rising resistance (ESBLs, CRE) is a major concern.

⭐ E. coli typically shows a green metallic sheen on EMB agar.

Enterobacteriaceae reactions on MacConkey & TSI agar

High‑Yield Points - ⚡ Biggest Takeaways

Gram-negative rods, oxidase-negative, glucose fermenters.

Lactose fermentation (e.g., E. coli - LF; Salmonella - NLF) on MacConkey agar is key.

Major pathogens: E. coli (UTI), Salmonella (typhoid), Shigella (dysentery), Klebsiella (pneumonia).

Serotyping based on O (somatic), H (flagellar), K (capsular) antigens.

Most are motile; exceptions: Klebsiella, Shigella, Yersinia (KSY).

IMViC tests are vital for biochemical differentiation.

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