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• Neisseria


  Gram-negative cocci that usually occur in pairs (diplococci)
• Neisseria gonorrhoeae (gonococci) and Neisseria meningitidis (meningococci)


  • Exclusively pathogenic for humans and typically are found associated with or inside polymorphonuclear cells (PMNs)
• Typical Neisseria


  • Aerobic, Gram-negative, nonmotile diplococcus, approximately 0.8 µm in diameter
  • Individual cocci are kidney bean shaped; when the organisms occur in pairs, the flat or concave sides are adjacent
• Culture


  1. The various pathogenic and nonpathogenic Neisseria species can be differentiated by their ability to grow on sheep blood agar, chocolate agar, and selective agar media

  2. N. meningitis grows on sheep blood agar as well as selective media

  3. N. gonorrhoeae requires enriched chocolate agar and/or selective media for optimal growth

  4. Selective media contain vancomycin, colistin, and other inhibitory substances to suppress the growth of many of the commensal microorganisms
• Neisseria growth characteristics


  • They grow best under aerobic conditions, some Neisseria species (eg, N. gonorrhoeae) are capable of growing under anaerobic conditions as well
  • They produce acid but not gas by oxidation of various carbohydrates (not by fermentation!)
  • The oxidase test is a key test for identifying neisseriae
  • All Neisseria species, with the exception of N. elongata, are catalase positive
  • Meningococci and gonococci grow best on media containing complex organic substances, such as heated blood, hemin, and animal proteins, and in an atmosphere containing 5% CO2
  • They are rapidly killed by drying, prolonged exposure to sunlight, moist heat, and many disinfectants
  • They produce autolytic enzymes that result in rapid swelling and lysis in vitro at 25°C and at an alkaline pH
• Neisseria Gonorrhoeae (Gonococci)


  • Oxidize only glucose and differ antigenically from the other neisseriae
  • Gonococci that require arginine, hypoxanthine, and uracil (Arg−, Hyx−, and Ura− auxotype) tend to grow most slowly on primary culture
• Antigenic Structure of N. gonorrhoeae


  • Pili (Fimbriae): Hairlike appendages that enhance attachment to host cells and resistance to phagocytosis
  • Por protein: Extends through the gonococcal cell membrane, forms pores for nutrient entry
  • Opa Proteins: Function in adhesion of gonococci and attachment to host cell receptors
  • Rmp (Protein III): Antigenically conserved, associates with Por in pore formation
  • Lipooligosaccharide: Endotoxic effects cause ciliary loss and mucosal cell death
• • Gonococci that form opaque colonies are isolated from men with symptomatic urethritis and from uterine cervical cultures at midcycle
  • Gonococci that form transparent colonies are frequently isolated from men with asymptomatic urethral infection, from menstruating women, and from patients with invasive forms of gonorrhea
• Pathogenesis of gonorrhea


  • Gonococci attack mucous membranes of the genitourinary tract, eye, rectum, and throat, producing acute suppuration that may lead to tissue invasion and chronic inflammation and fibrosis
  • In men, causes urethritis with yellow, creamy pus and painful urination
  • Gonococcal bacteremia leads to skin lesions, tenosynovitis and suppurative arthritis
  • Gonococcal endocarditis is an uncommon but severe infection
  • Gonococcal ophthalmia neonatorum is an infection of the eye in newborns acquired during passage through an infected birth canal
• Diagnostic Laboratory Tests


  1. Specimens: Pus and secretions taken from various sites for culture and smear, blood culture for systemic illness

  2. Smears: Gram-stained smears of exudates typically reveal many diplococci within PMNs, providing a presumptive diagnosis

  3. Culture: Specimen streaked on enriched selective medium and incubated in 5% CO2 at 37°C, definitive species identification by carbohydrate utilization
• Neisseria Meningitidis


  • Capsular polysaccharides: The six most important serogroups associated with disease in humans are A, B, C, X, Y, and W-135
  • Outer membrane consists of proteins and LPS that play major roles in virulence, including porin proteins (Por A and Por B) and opacity proteins (Opa)
• Gram-stained smears


  Showing typical neisseriae within polymorphonuclear leukocytes or extracellularly
• Colonies of N. meningitidis


  Gray, convex, and glistening, with entire edges
• N. meningitidis


  • Positive oxidase test together with a Gram-stain showing Gram-negative diplococci provides presumptive organism identification
• Identification of N. meningitidis


  1. Carbohydrate oxidative reactions

  2. Agglutination with type-specific or polyvalent serum
• Serology


  Antibodies to meningococcal polysaccharides can be measured by latex agglutination or hemagglutination tests or by their bactericidal activity
• The nasopharynx is the portal of entry for N. meningitidis
• N. meningitidis may form part of the transient microbiota without producing symptoms and/or disease
• Invasive meningococcal disease (IMD) occurs in only a small number of individuals who acquired the organism and are transient carriers
• Infants and adolescents have the highest incidence of IMD in developed countries
• Progression of IMD


  1. Meningococcal bacteremia

  2. Meningitis

  3. Sepsis (meningococcemia)
• Meningitis


  • Acute inflammation of the meninges, with thrombosis of blood vessels and exudation of polymorphonuclear leukocytes, so that the surface of the brain is covered with a thick purulent exudate
• Fulminant meningococcemia


  • High fever and a hemorrhagic rash
  • Disseminated intravascular coagulation and ultimate circulatory collapse
  • Bilateral hemorrhagic necrosis of the adrenal glands with subsequent adrenal failure (Waterhouse-Friderichsen syndrome)
• The exact mechanisms that transform an asymptomatic colonization of the nasopharynx into meningococcal bacteremia, subsequently leading to meningococcemia and meningitis, are not very well understood
• Typical specimens for isolation of N. meningitides


  • Blood for culture
  • Cerebrospinal fluid (CSF) for smear and culture
  • Puncture material or biopsies from petechiae for smear and culture
  • Nasopharyngeal swab cultures for carrier surveys
• Neisseriae are inhibited by certain toxic factors present in media and polyanethole sulfonate (anticoagulant) present in commercial blood culture broths
• CSF specimens are plated on sheep blood agar and chocolate agar and then incubated at 37°C in an atmosphere of 5% CO2
• A MTM agar favors the growth of neisseriae, inhibits many other bacteria, and is used for nasopharyngeal cultures
• Sporadic cases of zoonotic infections


  • Mainly from cattle, but also from vegetables washed in contaminated water
  • Low infecting dose
  • Acquired by eating undercooked contaminated meat and vegetables
  • Damage gut endothelium, resulting in haemorrhagic colitis
  • HUS occurs in about 5% of patients, which results in renal failure, oliguria, thrombocytopenia
• Diarrhoea caused by other E. coli


  • Enteropathogenic (EPEC): cause of infantile diarrhoea
  • Enterotoxigenic (ETEC): travellers' diarrhoea, non-invasive
  • Enteroinvasive (EIEC): causes dysentery-like illness
  • Enteroaggregative (EAEC): watery diarrhoea without fever
• Enterobacteriaceae


  • Commonly present in large intestine
  • Non spore forming, Non Acid fast, Gram-ve bacilli
  • A complex family of organisms
  • Some are non-pathogenic, a few are highly pathogenic
  • Some commensals turn out to be pathogenic, as in UTI after catheterization
• Characters of Enterobacteriaceae


  • Gram-negative rods
  • Ferment glucose with acid production
  • Reduce nitrates into nitrites
  • Oxidase negative
  • Facultative anaerobic
  • Motile except Shigella and Klebsiella
  • Non-capsulated except Klebsiella
  • Non-fastidious
  • Grow on bile containing media (MacConkey agar)
  • Non-spore forming
• Classification of Enterobacteriaceae


  • Lactose fermenters: E. coli, Klebsiella
  • Late lactose fermenters: Shigella sonnei
  • Non Lactose fermenters: Salmonella, Shigella
• Escherichia coli


  • Gram-negative (-ve) rod-shaped bacteria
  • 1-3 x 0.4-0.7 µm in size and 0.6 to 0.7 µm in volume
  • Arranged singly or in pairs
  • Motile due to peritrichous flagella, some strains are non-motile
  • Some strains may be fimbriated with type 1 fimbriae
  • Some strains have a polysaccharide capsule
  • Non-sporing
  • Thin cell wall with only 1 or 2 layers of peptidoglycan
  • Facultative anaerobes
  • Growth occurs over a wide range of temperatures from 15-45°C
• Antigenic Structure of E. coli

  • Heat Stable Lipopolysaccharide (LPS)
  • H or Flagellar Antigen
  • O or Somatic Antigen
  • K or Capsular Antigen
  • F or Fimbrial Antigen
• H or Flagellar Antigen


  Heat and alcohol labile protein, present on the flagella, genus specific, present as monophasic, 75 'H' antigens have been recognized
• O or Somatic Antigen


  Heat stable, resistant to boiling up to 2 hrs. 30 minutes, occur on the surface of the outer membrane, an integral part of the cell wall, 173 'O' antigens have been recognized
• K or Capsular Antigen


  Heat labile, acidic polysaccharide antigen presents in the envelope, boiling removes the K antigen, inhibit phagocytosis, 103 'K' antigens have been recognized
• F or Fimbrial Antigen


  Heat labile proteins, present in the fimbriae, K88, K99 antigens
• Pathogenicity of E. coli


  • Most infections are endogenous, associated with a variety of diseases including gastroenteritis and extra-intestinal infections such as UTIs, meningitis, and sepsis, a multitude of strains are capable of causing disease, some serotypes associated with greater virulence