Skip to main content
Microbiology 3% exam weight

Topic 4

Part of the FMGE study roadmap. Microbiology topic microb-004 of Microbiology.

→ See in Roadmap Build 1-Month Plan

Topic 4: Gram-Negative Cocci, Bacilli & Enterobacteriaceae

Introduction

Gram-negative bacteria are responsible for a vast array of human infections, from meningitis and gonorrhea to typhoid, dysentery, and plague. This chapter covers the most clinically important gram-negative organisms for FMGE: Neisseria species, ** Haemophilus**, the Enterobacteriaceae family, and other clinically significant gram-negative rods.

Neisseria

Neisseria meningitidis (Meningococcus)

Morphology: Gram-negative, kidney bean-shaped diplococci; intracellular diplococci seen in CSF neutrophils

Classification: Serogroups based on capsular polysaccharide: A, B, C, W135, X, Y, Z (most common in India: A and C; globally: B and C)

Diseases:

  • Meningitis — most feared presentation; rapid onset, petechial rash, DIC
  • Meningococcemia — without meningitis; high fever, purpuric rash, shock
  • Waterhouse-Friderichsen syndrome — bilateral adrenal hemorrhage with meningococcemia; rapidly fatal

Laboratory diagnosis:

  • Gram stain of CSF — gram-negative diplococci within WBCs
  • Culture on chocolate agar (not blood agar — requires factors X and V from heated blood)
  • Throat swab culture
  • Latex agglutination for capsular antigen detection

Prevention: Meningococcal vaccines (conjugate ACYW, polysaccharide A+C). Rifampicin or ciprofloxacin prophylaxis for close contacts.

Neisseria gonorrhoeae (Gonococcus)

Morphology: Gram-negative kidney bean-shaped diplococci; obligate human pathogen — no animal reservoir

Transmission: Sexual contact; vertical transmission (ophthalmia neonatorum)

Diseases:

  • Urethritis/cervicitis — purulent discharge, dysuria
  • Disseminated gonococcal infection (DGI) — arthritis-dermatitis syndrome
  • Ophthalmia neonatorum — conjunctivitis in newborns; preventable with silver nitrate or erythromycin eye drops

Laboratory diagnosis:

  • Gram stain of discharge — gram-negative intracellular diplococci (classic finding in male urethritis)
  • Culture on chocolate agar with antibiotics (Thayer-Martin medium — selective for Neisseria)
  • Nucleic acid amplification test (NAAT) is most sensitive

Treatment: Ceftriaxone (single IM dose) + azithromycin (for concurrent chlamydia). Increasing azithromycin resistance reported.

Haemophilus influenzae

Morphology: Gram-negative coccobacillus; pleomorphic; requires factors X (hemin) and V (NAD) for growth

Culture: Chocolate agar — small, translucent, iridescent colonies. Satellitism phenomenon — tiny colonies near S. aureus streaks (the staph provides V factor).

Type b (Hib) — Most Pathogenic Serotype:

  • Meningitis — most common cause of bacterial meningitis in children 6 months to 5 years (now reduced by Hib vaccine)
  • Epiglottitis — life-threatening; “thumb sign” on lateral neck X-ray
  • Pneumonia, otitis media, sinusitis

Laboratory: Quellung reaction with type-specific antisera detects capsular polysaccharide. Countercurrent immunoelectrophoresis (CIEP) can detect Hib antigen in CSF.

Prevention: Hib conjugate vaccine (part of universal immunization program in India) — dramatically reduced Hib meningitis and epiglottitis.

Enterobacteriaceae

This large family includes many clinically important gut-derived gram-negative rods. They are lactose fermenters or non-fermenters.

Escherichia coli

Characteristics: Lactose fermenter; motile (peritrichous flagella); indole-positive; lysine decarboxylase-positive; citrate-negative (IMViC: ++ – –); Eosin methylene blue (EMB) agar shows metallic green sheen

Pathogenic strains:

  • EPEC (Enteropathogenic) — infantile diarrhea; attaching and effacing lesions
  • ETEC (Enterotoxigenic) — traveler’s diarrhea; heat-labile (LT) and heat-stable (ST) enterotoxins; watery diarrhea
  • EHEC (Enterohemorrhagic) — O157:H7; Shiga toxin; bloody diarrhea, HUS (hemolytic uremic syndrome) — do NOT give antibiotics (increases toxin release)
  • EIEC (Enteroinvasive) — dysentery-like; invades colonic mucosa
  • EAEC (Enteroaggregative) — persistent diarrhea in children

Klebsiella pneumoniae

Characteristics: Lactose fermenter; large, mucoid (hypercapsulated) colonies; non-motile; very prominent polysaccharide capsule

Diseases: Hospital-acquired pneumonia (especially in alcoholics and diabetics), UTI, bacteremia, necrotizing pneumonia (currant jelly sputum)

Proteus Species

Characteristics: Non-lactose fermenter; motile; produces urease (liberates ammonia → alkaline urine); swarming motility on blood agar; associated with struvite (staghorn) kidney stones

  • Proteus mirabilis — most common; UTI, wound infections
  • Proteus vulgaris — indole-positive (mirabilis is indole-negative)

Shigella Species

Characteristics: Non-lactose fermenter; non-motile; does not produce gas from glucose; highly communicable (low infectious dose: 10–100 organisms)

Species and associated disease:

  • S. dysenteriae — produces Shiga toxin; most severe dysentery; can cause HUS
  • S. flexneri, S. boydii, S. sonnei — less severe dysentery

Laboratory: Gram-negative rods; culture on MacConkey agar (colorless), SS agar, XLD agar (red colonies)

Transmission: Fecal-oral; person-to-person spread; common in children

Salmonella Species

Characteristics: Non-lactose fermenter; motile (peritrichous flagella); produces H₂S (on triple sugar iron and Wilson-Blair media)

  • Salmonella typhi — typhoid fever (enteric fever); stepwise fever, rose spots, hepatosplenomegaly, relative bradycardia
  • Salmonella enteritidis, S. choleraesuis — nontyphoidal salmonellosis; gastroenteritis

Laboratory diagnosis: Blood culture (for typhoid — biphasic medium like Rao’s bile broth); stool culture (Widal test is supportive but not definitive — significant rise in O and H titers)

Prevention: Typhoid vaccines — oral (Ty21a) and injectable (Vi polysaccharide vaccine — single dose, revaccination every 3 years)

Pseudomonas aeruginosa

Characteristics: Non-lactose fermenter; obligate aerobe (oxidase-positive); produces blue-green pigments (pyocyanin, pyoverdin); grape-like odor; grows at 42°C (distinguishes from other non-fermenters)

Diseases: Hospital-acquired pneumonia (especially in CF and burn patients), ventilator-associated pneumonia, catheter-associated UTI, malignant otitis externa, hot tub folliculitis, burn wound infections, osteochondritis of foot (puncture through shoe)

Biofilm formation on prosthetic devices makes eradication difficult.

FMGE High-Yield Points

  • Meningococcus = gram-negative diplococci in CSF; petechial rash; Waterhouse-Friderichsen
  • Gonococcus = gram-negative intracellular diplococci; STI; purulent discharge
  • Hib = type b capsular polysaccharide; causes meningitis in children (now vaccine-preventable)
  • E. coli — most common cause of UTI; O157:H7 = bloody diarrhea + HUS (no antibiotics)
  • Klebsiella = mucoid colonies; “currant jelly” sputum in pneumonia
  • Proteus = urease-positive; swarming motility; associated with staghorn calculi
  • Shigella = very low infectious dose; bloody diarrhea; person-to-person spread
  • Salmonella typhi = stepwise fever; rose spots; H₂S production; Widal test
  • Pseudomonas = oxidase-positive; blue-green pigments; grows at 42°C; nosocomial

⚡ Exam tip: Non-lactose fermenter + H₂S producer + motile = think Salmonella or Proteus. Non-lactose fermenter + H₂S negative + non-motile = Shigella.