Neisseria on Blood Agar: Introduction, Culture characteristics, Pathogenecity, Laboratory Diagnosis and Treatment

Neisseria on blood agar

 Neisseria gonorrhoeae Introduction

Neisseria gonorrhoeae, also called Gonococcus which causes the sexually transmitted disease(STD) gonorrhea. First described by Albert Ludwig Sigesmund Neisser  in 1879 in gonorrheal pus. It resembles meningococci very closely in many properties.

  • Colony characteristics of Neisseria gonorrhoeae on 5% sheep blood agar (BAP) are as follows-
  • Small
  • Round
  • grey
  • white
  • opaque as shown above picture.
  • They are easily emulsifiable.
  • It produces four types of colonies and they are T1, T2, T3, and T4.
  •  T1 and T2: Small brown colonies and possess pili. They are autoagglutinable and virulent strains. On repeated subcultures, they change to T3 and T4 respectively. They are also known as P+ and P++respectively.
  •  T3 and T4: They are larger, granular, non-pigmented colonies and non-pi
  • Piliated. They form smooth suspensions and are avirulent. They are also named as P-.

Note: few large white colonies  on blood agar are because of contamination

Scientific Classification

Domain: Bacteria

Phylum: Proteobacteria

Class: Betaproteobacteria

Order: Neisseriales

Family: Neisseriaceae

Genus: Neisseria

Note: Medically important species of this genus are  Neisseria gonorrhoeae  (gonococcus) and Neisseria meningitidis ( meningococcus). But here we concern with gonococcus only.


Gram-negative oval cocci arranged in pairs i.e. diplococci with adjacent sides concave ( pear or bean-shaped). In smear from purulent material, they are intracellular within polymorphonuclear cells( polymorphs), some cells containing as many as a hundred cocci as shown below.

 Culture Characteristics of Neisseria

This gonococcus is fastidious and thus does not grow on ordinary culture media like nutrient agar. It is aerobic but may grow anaerobically also. The optimum temperature for growth is 35-36°C and the pH is 7.2-7.6. It is essential to provide 5-10% CO2.  Media used: a) Non-selective media: Chocolate agar, Mueller-Hinton agar Modified New York City medium b) Selective media: Thayer Martin medium with antibiotics (Vancomycin, Colistin, and  Nystatin). Vancomycin and colistin are antibacterial agents that kill Gram-positive and Gram-negative bacteria respectively ignoring the Neisseria while nystatin is an antifungal drug that makes the medium selective. Colony morphology is shown above.

Biochemical reactions

  • Oxidase test: Positive

  • Gonococci ferment only glucose but not maltose.

Antigenic structure  of Neisseria

  1. Pili: These are hair-like structures extending from the surface. They enhance the attachment of the organism to host cells and resist phagocytosis.
  2. Lipopolysaccharide: Endotoxic.
  3. Outer membrane proteins: 3 types a) Protein I (por)- it is a porin & helps in adherence. b) Protein II (opa)- helps in adherence. c) Protein III (rmp)- it is associated with protein I.
  4. Other Protein
    IgA1 protease: Splits and inactivates IgA.


Gonococci adhere to epithelial cells of the urethra or other mucosal surfaces through pili penetrate through the intercellular space reach the subepithelial connective tissue and causes inflammation which leads to clinical manifestations. The incubation period is 2-8 days. Source of infection: 1. Asymptomatic carriers 2. Patients Mode of infection: 1. Venereal infection (sexual contact) 2. Non-venereal infection. In men: The disease starts as acute urethritis with a mucopurulent discharge that extends to the prostate, seminal vesicles, and epididymis. In some, it may become chronic urethritis leading to stricture formation. The infection may spread to the periurethral tissues, causing abscesses and multiple discharging sinuses while in women: The initial infection is urethritis and cervicitis but vaginitis does not occur in adult females ( vulvovaginitis can occur in prepubertal girls). The infection may extend to Bartholin’s glands, endometrium, and fallopian tubes causing Pelvic Inflammatory Disease (PID). Rarely peritonitis may develop with perihepatic inflammation (Fitz-Hugh-Curtis syndrome). Common in both the sexes: Proctitis, pharyngitis, conjunctivitis, bacteremia which may lead to metastatic infection such as arthritis, endocarditis, meningitis, pyemia, and skin rashes. In neonates: Ophthalmia neonatorum (nonvenereal gonococcal conjunctivitis in the newborn) results from direct infection during passage through the birth canal.

 Laboratory Diagnosis  of Neisseria

  • Specimens: A) In men: a) Acute infection- Urethral discharge b) Chronic infection- i) Morning drop ii) Discharge collected after prostatic massage iii) Centrifuged deposit of urine

B) In women: i) Urethral discharge ii) Cervical swabs

C) In both the sexes: Blood, CSF, synovial fluid, throat swab, rectal swab, and material from skin rashes.

  • Transport: If there is a delay in processing then the specimens should be sent in “ Stuart’s transport medium”.
  • Direct microscopy: Gram staining: Smear provides presumptive evidence of gonorrhea in men. Gram-negative diplococci are found. But it is unreliable in women because some of the genital normal flora have similar morphology. Fluorescent antibody stain of gonococcus can also be observable under the fluorescence microscope.
  • Culture: Media used: Colony morphology: Gram’s smear: Reveals Gram-negative cocci in pairs with adjacent sides concave.
  • Biochemical reactions: 1)  Oxidase test: Positive2) Gonococci ferment only glucose but not maltose.
  • Serology:  Serology tests can be performed using the following tools like complement fixation test, precipitation, passive agglutination, immunofluorescence, and radioimmunoassay.


Previously Penicillin was the drug of choice but resistance developed rapidly. Penicillin resistance is due to the production of the penicillinase enzyme and the strains are called penicillinase-producing Neisseria gonorrhoeae (PPNG). Now ceftriaxone or ciprofloxacin plus doxycycline or erythromycin are useful drugs.


Gonorrhea is an exclusively human disease. The only source of infection is a human carrier or less often a patient.  Asymptomatic carriage in women makes them a reservoir to spread the infection among their male contact. Gonorrhea is a venereal disease (STD).

Prophylaxis of Neisseria

Early detection of cases, tracing of contacts, health education, General measures, and vaccination has no role in prophylaxis.

 Non-Gonococcal  (Non-specific)) Urethritis

Urethritis is due to causative agents other than gonococcus.

Causative agents


  • Chlamydia trachomatis ( most common)
  • Mycoplasma hominis
  • Ureaplasma urealyticum
  • Gardnerella vaginalis
  • Acinetobacter lwoffi


  • Herpes simplex
  • Cytomegalovirus


  • Candida albicans

Parasitic ( protozoal)

  • Trichomonas vaginalis NGU can be a part of Reiter’s syndrome- a clinical condition characterized by urethritis, arthritis & conjunctivitis.
  • Tetracycline is the effective drug for both Chlamydia trachomatis and Ureaplasma urealyticum infections.

Commensal Neisseria

Commensal of Genus Neisaseria few species are  N. lactamica N. cinerea N. flavescens N. sicca and N. subflava.

Furthher Readings

  1. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  2. Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  3. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr, and Sommers H.M.
  4. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  5. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  6.  Manual of Clinical Microbiology. Editors: P.R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover and R. H. Yolken, 7th ed 2005, Publisher ASM, USA
  7.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
  8. Topley & Wilsons’ Principle of Bacteriology, Virology, and immunology. Editors: M.T. Parker & L.H. Collier, 8th ed 1990, Publisher Edward Arnold publication, London.
  9. Medical Microbiology-The Practice of Medical Microbiology Vol-2-12th Edn. –Robert Cruickshank
  10. District Laboratory Practice in  Tropical Countries  –  Part-2-   Monica Cheesebrough-   2nd Edn Update
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