General Health

Morphology and Characteristics of Gram-Negative Cocci

Neisseria Meningitidis

It is a strict human pathogen and causes infection from the nasopharynx which reached the brain through the olfactory nerve or blood. It causes pyogenic meningitis and meningococcal septicemia. Meningitis is an infection of the meninges due to bacterial infection.

Symptoms of high fever, intense headache, loss of appetite, the rigidity of muscles especially neck intolerance to light and sound which can cause death, patechial rashes are formed in meningococcal septicemia due to hemorrhage by endotoxin.


  • It is a gram-negative cocci.
  • It may be oval or spherical measuring 0.6 to 0.8 micron in diameter.
  • They are arranged in pair with adjacent sides flattened.
  • They are non-sporing, non-motile but capsulated.
  • The cocci are present in extracellular spaces as well in neutrophils.

Cultural Characteristics:

  • It is aerobic at grows at 37° C and pH 7.4 with 5-10% Co2.
  • Growth occurs in media enriched with blood serum or ascitic fluid.
  • Ascitic fluid that is blood agar media, chocolate agar media, mueller hinton agar.
  • In, broth mild turbidity appear after 24 hours at 37°C.
  • In blood agar media colonies are grey, moist, smooth, round and convex with weak hemolysis.
  • Selective media are:

Biochemical Characteristics

  • Catalase positive
  • Oxidase positive
  • Glucose and maltose are fermented with production of acid and no gas.

Laboratory Diagnosis

  • Specimen: Nasopharyngeal swab, blood, CSF by lumbar puncture and scrapping from patechial lesion can be taken. This can be studied further by its morphology and biochemical test.
  • Staining: On gram staining, it shows gram-negative red-colored diplococci.
  • Blood Culture: Blood is collected by vein puncture and injected into blood culture bottle containing glucose broth or sodium taurocholate broth it is then incubated at 35° for 4 to 7 days. Subcultures are made on blood agar or chocolate agar. Again plates are incubated with 5 to 10% Co2 and identified by cultural characteristics, gram staining and biochemical reaction.

Method of CSF examination

The CSF is turbid and contains pus cells it can be divided into 3 portions:

  • The first portion of the fluid is centrifuged and examined by gram staining pink to red color diplococci in seen inside neutrophils and extracellular spaces.
  • The second portion is inoculated on blood agar of chocolate agar with 5 to 10% Co2.
  • After 24 hours of incubation, meningococci can be identified on the basis of morphology and biochemical reaction.
  • If the infection is very less and cannot be detected by direct methods the third portion of the sample is incubated overnight and then subculture on chocolate agar.


It is done on the meninges, that is surface of the brain and spinal cord within 12 hours of death after smear and culture meningococci can be identified.

Neisseria Gonorrhea

It is also an intracellular human parasite that affects the genital mucous living of both male and female reproductive organs with pus discharge it causes a disease called gonorrhea which is characterized by urethritis, cervicitis, salpingitis, epididymitis, pelvis inflammatory disease (PID) and neonatal conjunctivitis.


  • It is gram-negative diplococci.
  • It may be oval or spherical measuring 0.6 to 0.8 micron in diameter.
  • The adjacent sides are concave and arranged in pair.
  • These are non-motile, non-sporing but capsulated.
  • It is found predominantly within neutrophils.
  • It has pili on the surface which helps in attachment to host cell and resist phagocytosis.

Cultural Characteristics

  • It is aerobic and facultative anaerobe.
  • It grows best at 37°C and a pH 7.4.
  • The addition of 5 to 10% of Co2 is essential for growth.
  • It broth the growth is only on the surface with no turbidity.
  • An enriched media with blood or serum is required for growth. For example, blood agar media, and chocolate agar media.
  • On, chocolate agar media the colonies are small, round, convex, shiny, and grey in color.
  • Thayer martin media is a selective media that inhibits the growth of other contaminants as it contains antibiotics.

Biochemical Reaction

  • Catalase positive
  • Oxidase positive
  • Glucose is fermented with acid production and no gas.

Laboratory Diagnosis

  1. Hematological investigation:
  • TLC shows leukocytosis
  • DLC shows an increase in the number of neutrophils.

2. Bacteriological investigation:

  • Specimen: Specimen of urethral discharge or pus can be taken with the help of a cotton swab soaked in saline, and centrifuged deposit of urine can be examined in the absence of urethral discharge. Cervical discharge is taken with the help of a cotton swab through the endocervical canal in case of cervicitis.
  • Staining: Gram staining from pus discharge shows gram-negative diplococci it can be seen inside polymorphs as well as extracellular spaces. The fluorescent antibody technique is a specific and sensitive method of identification.
  • Culture: Culture on blood agar and chocolate agar media can be done with 10% Co2 in case of delay swab should be transported to the laboratory in stuart transport media. In case of mixed infection, selective media like Thayer Martin should be used.
  • Serological Test: The sample taken for the serological test is serum and genital fluid the serological test are complement fixation tests, flocculation, immunofluorescence, and ELISA.


Yash Batra

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