Brucella: Morphology, Cultural Characteristics, Biochemical Reactions, Prophylaxis

The genus Brucella consist of very small, nonmotile, aerobic, Gram negative coccobacilli those grow poorly on ordinary media and have little or no fermentative powers. They are strict parasite of animals and may also infect humans.


Brucellae are coccobacilli or short rods 0.5 – 0.7 x 0.6 – 1.5 micrometer in size. They are arranged singly or in short chains. The cells are so small that they may be mistaken for cocci. In older cultures, irregular forms appear. they are non motile, non capsulated, and non sporing. They are Gram negative and non acid fast.

Cultural Characteristics

Brucellae are strict aerobes and do not grow anaerobically. Br. abortus is capnophilic (Capnophiles are microorganisms that thrive in the presence of high concentrations of carbon dioxide (CO 2). ) many strains requiring 5 – 10% CO2 for growth. The optimum temperature is 37 C (range 20 – 40 C) and pH 6.6 – 7.4.

They may grow on simple media, though growth is slow and scanty. Growth can be improved by adding serum or liver extraterrestrial.

Liver infusion media were widely used for the cultivation of brucellae.

The media employed currently serum dextrose agar, or tryptophan agar. The addition of bacitracin, polymyxin and cycloheximide to the above media makes them selective.

In liquid media, growth is uniform, and a powdery or viscous deposit is formed in old cultures.

On solid media, colonies are small, moist, translucent and glistening. Mucoid, smooth and rough types of colonies appear, associated with changes in antigenic structure and virulence.

Biochemical Reactions

No carbohydrate are ordinarily fermented, though they possess oxidative capacity. Brucellae are catalase positive, oxidase positive (except for Br. Neotomae and Br. Ovis which are oxidase negative) and urease positive. Nitrates are reduced to nitrites. Citrate is not utilized. Indole is not produced and MR and VP tests are negative.


Brucellae are destroyed by heat at 60 C in 10 minutes and by 1% phenol in 15 minutes. They can be killed by pasteurization. They may survive in soil and manure for several weeks.

They remain viable for 10 days in refrigerated milk, one month in ice cream, four months in butter and for varying periods in cheese depending on its pH. They may also survive for many weeks in meat. 

They are sensitive to direct sunlight and acid, and tend to die in buttermilk.

Br. melitensis may remain alive for six days inn urine, six weeks in dust and ten weeks in water.


All three major species of brucellae are pathogenic to human beings. Br. melitensis is the most pathogenic to humans, Br. abortus and Br. suis is of intermediate pathogenicity.

The incubation period of infection of brucella is usually about 10 – 30 days, but may sometimes be very prolonged.

Human infection of brucella may be of three types:

  1. Latent infection with only serological but no clinical evidence;
  2. Acute or subacute brucellosis; and
  3. Chronic brucellosis.


As the majority of human infections are acquired by consumption of contaminated milk, prevention consists of checking brucellosis in dairy animals. In many advanced countries, this is achieved by the detection of infected animals, their elimination by slaughter and the development of certified brucella – free herds. Pasteurization of milk is an additional safeguard.

Vaccines have been developed for use in animals. Br. abortus strain 19 vaccine is protective in cattle. No suitable vaccine is available for human use.


The usual regimen is a combination of doxycycline for 45 days with streptomycin IM daily for the first 2 weeks in adults, and in children contrimoxazole along with rifampicin or gentamycin.

Reference: The Text Book Of Microbiology

Gaurav Singh

Editor in Chief Medical Microbiology & Recombinant DNA Technology (RDT) Labs - RDT Labs Magazine

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