medically important gram negative

• Fastidious Gram Negative Bacteria


  Bacteria that are difficult to grow in the laboratory and have specific growth requirements
• Fastidious Gram Negative Bacteria


  • Haemophilus
  • Neisseria
  • Legionella
  • Bordetella
• Haemophilus


  • Microman12345 / CC BY-SA [Image] (2019). Available from: https://commons.wikimedia.org/wiki/File:Haemophilus_influenzae_sputum_1000x_edited.jpg
• Haemophilus influenzae


  Requires X-factor (haemin) and V-factor (NAD) for growth
• Lab Diagnosis of H. influenzae

  1. Candle jar/Anaerobic jar/Gas pack

  2. API (Analytical Profile Index) biochemical tests
• API Test


  Classification of bacteria based on biochemical tests
• Neisseria


  Contains 2 notable human pathogens: Neisseria meningitidis (causes meningitis and meningococcaemia) and Neisseria gonorrhoeae (causes gonorrhoea)
• Neisseria meningitidis Serotypes


  • A
  • B
  • C
  • W135
  • X
  • Y
• Neisseria meningitidis


  • Type A most prevalent in Africa and Asia, Type B & C most predominant in North America and Europe
• Lab Diagnosis of Neisseria meningitidis

  1. Smear and culture of blood and spinal fluid samples

  2. Culture on chocolate agar, Thayer Martin agar, New York City agar at 35-36.5°C with 5% CO2

  3. Biochemical tests: Oxidase+, API NH
• Legionella


  Legionella pneumophila (and other legionellae) causes pneumonia (Legionnaires' disease)
• Legionella spp. are Gram-negative bacilli that stain faintly with standard Gram stain</b>
• Legionellae in lung biopsy sections do not stain by H&E so need special methods e.g. Dieterle silver impregnation stain
• Prevention and Treatment


  • Prevention = Chemoprophylaxis and immunization
  • Treatment = Penicillin or ceftriaxone (cephalosporin)
• Either rifampin or ciprofloxacin can be used for prophylaxis in people who have had close contact with an infected person
• NHS vaccination schedule: https://www.nhs.uk/conditions/vaccinations/nhs-vaccinations-and-when-to-have-them/
• Fastidious Gram Negative Bacteria
• Legionella


  Causes pneumonia
• Legionella spp.


  • 16 serogroups (1 = most pneumonia cases)
  • ~ 400 cases of legionellosis per year in UK (over ½ associated with foreign travel)
  • 10-15% mortality in healthy individuals
• Legionnaires' disease


  Outbreak of pneumonia among people attending the American Legion convention in Philadelphia in 1976
• Legionella spp.


  • Gram-Negative Bacilli, but stain faintly with standard Gram stain
  • Legionellae in lung biopsy sections do not stain by H&E so need special methods e.g. Dieterle silver impregnation stain
• Legionellae widely distributed in environmental water sources = ponds, hot and cold water systems, air conditioning units
• Legionellae can replicate to large #s in free-living amoebas in these water sources
• The amoebas also enhance the survival of Legionellae
• Under adverse environmental conditions the amoebas encyst
• Legionella spp. - Clinical findings

  • From mild influenza-like illness to a severe pneumonia
  • Cough is a prominent symptom, sputum is frequently scanty and non-purulent
  • Hyponatremia
  • Can be fatal
  • Atypical pneumonia; need to distinguish from others
  • Pontiac fever = mild form of legionella infection
• Legionella spp. - Lab diagnosis

  • Sputum Gram stains reveal many neutrophils but no bacteria
  • Buffered charcoal yeast agar in aerobic conditions in humidified incubator at 37°C
  • Indirect IF assay from convalescent serum
  • Urine antigen test for L. pneumophilia (only serogroup 1)
  • Legionella direct IF test environ-mental samples
  • Latex agglutination test
  • The cold-agglutinin titer does not rise
• Azithromycin or erythromycin


  Treatment of choice for Legionella spp.
• Penicillins and cephalosphorins are less effective for treating Legionella spp.
• Prevention of Legionella spp.

  • Reducing cigarette and alcohol consumption
  • Eliminating aerosols from water sources
  • Reducing the incidence of Legionella in hospital water supplies by using high temperatures and hyperchlorination
  • There is no vaccine
• Bordetella pertussis - A Brief Overview
• Bordetella pertussis


  • Causes whooping cough (pertussis)
  • Small, coccobacillary, encapsulated GNB
  • B. pertussis was identified and isolated by Jules Bordet and Octave Gengou in 1906
  • Virulence factors: pertussis toxin, adenylate cyclase toxin, filamentous hæmagglutinin, pertactin, fimbria, and tracheal cytotoxin
  • Bordet-Gengou medium, aerobic, 37°C
  • Human pathogen
  • Transmitted by airborne droplets
  • Incubation period is 7–10 days on average (range 6–20 days)
  • Isolated from nasopharyngeal swabs taken during the paroxysmal stage
  • Produces multiple antigenic and biologically active products that are responsible for the clinical features of pertussis