Gram-Negative Bacteria

Gram-Negative Bacteria-

Gram-negative bacteria lose the crystal violet stain (and take the color of the red counterstain) in Gram's method of staining. This is characteristic of bacteria that have a cell wall composed of a thin layer of a particular substance (called peptidoglycan).

  1. Neisseria

The genus Neisseria is named after Albert Neisser, who discovered the organism, which causes gonorrhoea disease.

General Characters-

  • They are Gram negative, kidney-shaped cocci found in pairs.
  • They are non-motile, non-sporing and non-capsulated.
  • They require special substances in culture media such as blood or serum for growth.
  • They are aerobic and the germs die very quickly outside the body.
  • They are readily killed by usual chemical disinfectants.
  • They are transmitted by direct contact or air borne droplets and rarely by fomites or carriers.

Two species are pathogenic in human being-

  • Neisseria gonorrhoeae.
  • Neisseria meningitidis.

a. Niesseria gonorrhoeae-

 It is a diplococcus organism, the two cocci with their concave sides facing each other. The organism is a strict human pathogen and enters throuh the mucous membrane of urogenital tract or conjunctiva causing gonorrhoea and conjunctivitis.


It is a venereal disease, and is transmitted by sexual intercourse. The organisms attack urethra in both the sexes. In the female, the entire urogenital tract including cervix may be involved. The organisms may also invade the blood stream, and produce gonorrhoeal arthritis. It may even lead to sterility in females.


The gonococci also cause conjunctivitis in the newborn. The conjunctivitis is not a venereal disease but it is passed on from genital tract of the infected mother to the baby. When a baby is delivered from a mother with gonorrhoea, the baby’s eyes become infected during its passage through the vagina.

Bacteriological Investigations-

Examination of Gram stained smears and swabs from discharges of infected persons.

b. Neisseria meningitidis (Meningococcus)-

 It is also a diplococcus organism, the two cocci with their adjacent sides facing each other. The organisms are present in nasopharynx, conjunctiva, cerebrospinal fluid, joints of infected persons and nasopharynx of healthy carriers. These organisms from the mucous membrane of the upper respiratory tract reach the blood stream, and sometimes even reach the central nervous system. The organisms are strictly human pathogens and cause mainly two types of infections, viz. cerebrospinal meningitis and  meningococcal septicaemia (brain fever).

Bacteriological Investigations- Examination of cerebrospinal fluid, pus, nasopharyngeal swab and sputum.

2. Haemophilus-

General Characters

  • These are Gram negative, very small Bacilli, found either singly, in pairs or in chains.
  • They are non-motile, non-sporing and non-capsulated.
  • iii) They require a special substance, blood or serum, for growth.
  • They are aerobic.

The genus Haemophilus includes following pathogenic species-

1.  Haemophilus influenzae (Influenza bacillus)-

 These organisms were once thought to be the cause of influenza, which we know, now, is caused by a type of virus. They are found singly or in pairs. These pathogens enter through the mucous membrane of the respiratory tract, cause bronchitis, bronchopneumonia and sinusitis. In children they can cause meningitis. 

2. Haemophilus ducreyi-

These organisms occur singly, in pairs or in chains, and cause inflammation of the skin surrounding genital organs. The inflammation is caused by sexual intercourse.

Bacteriological Investigations-

 Examination of pus, sputum and cerebrospinal fluid.

3. Bordetella Pertussis

 General Characters-

  • These are very small Gram negative bacilli.
  • They are non-motile, non-sporing and non-capsulated, and require specific media for growth.
  • They are aerobic or facultative anaerobic.
  • They cause one of the most infectious bacterial disease known as whooping cough.

Whooping cough-Whooping cough bacilli are present in nasopharynx, and respiratory tract mucosa. They are present in enormous numbers in the ciliated lining of the bronchi and trachea; they secrete endotoxin, which causes irritation of the tissues. The cough persists for several weeks. This infection is transmitted by droplets and by fomites contaminated with nasopharyngeal secretion. Now-a-days, vaccine is administered in combination with diphtheria and tetanus toxoids to prevent whooping cough in children.

Bacteriological Investigations-Examination of postnasal and perinasal swab by culturing method.

4. Brucella-

General Characters-

  • They are Gram negative bacilli found singly or in chains.
  • They are non-motile, non-sporing and non-capsulated.
  • They grow in ordinary media, but in enriched media the growth is quicker.
  • They are strict parasites of animals, but may also infect human being.

There are three main species-

  • B. abortus (found in cattle).
  • B. suis (found in pigs)
  • B. melitensis (found in goats and sheep)

The three are pathogenic to human being. These three differ only in culture characteristics. There is no morphological or staining differentiation. All these species enter through mucous membrane of the intestine or skin, either by infected milk or their products, raw vegetables, water contaminated by faeces or urine, or vaginal discharges of infected animals. Organisms also enter indirectly through personal contact with infected animals. From the intestine, the organisms enter the blood stream, and cause brucellosis.

Bacteriological Investigation-Blood culture and serological tests are the most definite methods.

5. Pasteurella pestis (Plague Bacillus)

It was discovered by Yersin, so it is also called as Yersinia pestis.

General Characters-

  • They are Gram negative bacilli found either singly, in short chains or in small groups.
  • They are non-motile, non-sporing, surrounded by slime layer, and grow in ordinary media.
  • They are facultative anaerobes.
  • They have granules at the poles.

Bubonic plague-

 In this the lymph nodes become swollen with pus and bacilli. These swollen lymph nodes are called buboes. These nodes often rupture and the pus is highly infectious.

Septicaemic plague-

 Lymph nodes are not affected but the infection spreads through blood stream. There may be haemorrhages from the skin and mucous membrane. The blood and tissues of the infected patients are highly infectious.

Pneumonic plague-

 It is caused by inhalation of plague organisms. Any severe case of bubonic plague becomes pneumonic, and it is spread by droplet infection. It is highly contageous, and spreads through lymphatics causing haemorrhagic pneumonia.

Bacteriological Investigations-

 Examination of pus or pneumonic sputum or Gram stained blood smears.

6. Enterobacteria (Intestinal Bacteria)-

These organisms belong to family Enterobacteriaceae. The name of this family is derived from the fact that nearly all organisms belonging to this family, more or less, inhabit in the intestine of human being. These are all Gram negative non-sporing bacilli.. Infection occurs through contaminated food and water, fomites, flies and carriers.

Important members of this family-

a. Escherichia coli :

This species is named after Escherich. They are non-sporing, non-capsulated and motile bacilli with peritrichate flagella. Normally they are harmless in their normal habitat until they gain access to other sites or tissues. E.coli produces enterotoxin and causes intestinal infection (diarrhoea gastroenteritis) urinary tract infection often acquired in hospitals (cystitis, pyelitis, pylonephritis) and pyogenic infections (wound infections, abscess, deep infection, meningitis, peritonitis).

Bacteriological Investigations-

 Examination of faeces in diarrhoea and gastroenteritis, urine in urinary infections and pus in pyogenic infections.

b. Salmonella typhi and S. paratyphi

They are non-sporing, non-capsulated and motile bacilli with peritrichate flagella. They resemble E.coli so much that they can be differentiated from E.coli only by biochemical and serological tests. S.typhi (typhoid bacillus) causes typhoid fever and S. paratyphi causes paratyphoid fever. Typhoid bacilli chiefly multiply in the human body, and are excreted in faeces, urine and vomit. They can survive outside the body in water and foods polluted by the excreta of typhoid patients. In ice or ice-cream they can survive for several days. Several other species of Salmonella can also cause gastroenteritis and food poisoning.

Bacteriological Investigation- Examination of vomit, faeces, samples of food, urine, blood and in some cases, cerebrospinal fluid.

c. Shigella dysenteriae-

 They are short, non-sporing, non-capsulated and nonmotile organisms, causing bacillary dysentery also known as Shigella dysentery. These bacilli multiply in the intestine, and form superficial ulcers.

d. Klebsiella pneumonia- They are non-sporing, non-motile, short, capsulated bacilli, found in the normal intestinal tract. They cause wounds, respiratory and urinary infections, chronic bronchitis and other pyogenic infections. They are responsible for about 3% cases of pneumonia. One or more lobes of the lung may be involved.

7. Vibrio Cholerae

General Characters-

  • They are comma shaped Gram negative bacilli found singly or in S shaped/semicircular pairs.
  • They are actively motile with a single polar flagellum.
  • They are non-sporing and non-capsulated.
  • They are strictly aerobic and grow in ordinary media.

These organisms cause acute gastroenteritis or cholera in human beings. Stool of the patient is like rice water and has a fishy odour. It contains mucous flakes, epithelial cells and cholera germs, resulting in fluid loss and dehydration. Vomiting also starts along with the diarrhea.

Bacteriological Investigations-

Examination of hanging drops and smears prepared from faeces or rectal swabs. Stool is collected in alkaline medium. If it is to be delayed more than six hours, then it is collected in special medium known as VR fluid.

8. Pseudomonas Aeruginosa

General Characters-

  • They are partly flagellated, straight and slightly curved bacilli.
  • They grow aerobically and do not form spores.
  • They obtain their energy by anaerobic respiration.
  • They are aquatic bacteria that can cause opportunistic infection in human being giving rise to wound infection producing greenish blue pus and can even cause septicaemia.

Pseudomonas aeruginosa is one of the most important causes of nosocomial (hospital) infections. Equipments, bedpans, ointments, eye drops, lotions etc. are often contaminated with this organism.