The genus Brucella consists of small, aerobic, gram-negative coccobacilli that primarily cause brucellosis in animals and can be transmitted to humans through contact or consumption of infected products. Brucellae are classified into different species, with B. melitensis being the most pathogenic, and infections can present as latent, acute, or chronic brucellosis, often diagnosed through blood cultures and serological tests. Treatment typically involves a combination regimen of antibiotics over several weeks, particularly for severe cases.

1. Brucella

2. INTRODUCTION
• The genus Brucella consists of very small, non-motile, aerobic, Gram-
negative coccobacilli that grow poorly on ordinary media and have
little or no fermentative powers.
• They are strict pBrucellosis is a zoonosis, primarily affecting goats,
sheep, cattle, buffaloes, pigs and other animals and transmitted to
humans by contact with infected animals or through their products.
• The human disease was recognised along the Mediterranean littoral
from very early times and has been known under various names, such
as Mediterranean fever, Malta fever and undulant fever parasites of
animals and may also infect humans.

3. Morphology
• Brucellae are coccobacilli or short rods, 0.5-0. 7 x 0.6-1.5 µm in size,
arranged singly or in short chains.
• The cells are so small that they may be mistaken for cocci, as was
done by Bruce who called them Micrococcus melitensis.
• In older cultures, irregular forms appear. They are non-motile, non-
capsulated and non-sporing.
• They are Gram negative and non-acid fast. Bipolar staining is not
uncommon.

4. Cultural characteristics
• Brucellae are strict aerobes and do not grow anaerobically. B.abortus is
capnophilic, many strains requiring 5-10% CO2 for growth. The optimum
temperature is 37°C (range 20-40°C) and pH 6.6-7.4.
• Simple media: Growth is slow and scanty. Liver infusion media were widely used
for the cultivation of brucellae. The media currently employed are serum
dextrose agar, serum potato infusion agar, trypticase soy agar or tryptose agar.
The addition of bacitracin, polymyxin and cycloheximide to the above media
makes them selective.
• Liquid media: Growth is uniform, and a powdery or viscous deposit is formed in
old cultures.
• 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

5. Biochemical reactions
• No carbohydrates are ordinarily fermented, though they possess
oxidative capacity.
• Brucellae are catalase positive, oxidase positive (except for
B.neotomae and B.ovis which are negative) and urease positive.
• Nitrates are reduced to nitrites. Citrate is not utilised.
• Indole is not produced and MR and VP tests are negative

6. Resistance
• Brucellae are destroyed by heat at 60°C in 10 minutes and by 1 % phenol in
15 minutes.
• They are killed by pasteurisation. 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.
• B.melitensis may stay alive for six days in urine, six weeks in dust and ten
weeks in water

7. Antigenic structure
• The somatic antigens of brucellae contain two main antigenic
determinants, A and M, which are present in different amounts in the
three major species.
• B.abortus contains about 20 times as much A as M;
• B.melitensis about 20 times as much M as A.
• B.suis has an intermediate antigenic pattern.
• Absorption of the minor antigenic component from an antiserum will
leave most of the major antibody component, and such absorbed A
and M monospecific sera are useful for species identification by the
agglutination test

8. Classification
• Brucellae may be classified into different species, based on their CO2
requirements, H2 S production, sensitivity to dyes (basic fuchsin and
thionin), agglutination by monospecific sera, phage lysis and oxidative
metabolic tests with amino acids and carbohydrates.
• The two main species are B.melitensis and B.abortus.
• Many biotypes have been recognised in these species.
• B.suis strains that produce H2S are known as 'American' strains and
those that do not as 'Danish' strains.

9. Pathogenicity
• All three major species of brucellae are pathogenic to human beings,
B.melitensis is the most pathogenic, B.abortus and B.suis being of intermediate
pathogenicity.
• Brucella is primarily an intracellular pathogen affecting the reticuloendothelial
system.
• This accounts for its refractoriness to chemotherapy and the co-existence of
viable bacilli with high levels of circulating antibodies.
• The lipopolysaccharide component of the Brucella cell wall is a virulence factor.
• Organisms from the infected animal enter the human body through a wound,
the conjunctiva, by inhalation or by ingestion of products from infected animals
The incubation period is usually about 10-30 days, but may sometimes be very
prolonged

10. • The brucellae spread from the initial site of infection through lymphatic
channels to the local lymph glands, in the cells of which they multiply.
• They then spill over into the bloodstream and are disseminated
throughout the body.
• They have a predilection for the placenta, probably due to the presence in
it of erythritol, which has a stimulating effect on brucellae in culture.
• Fever, sweats and extreme fatigue occur 2-4 weeks after initial infection.
• Human infection may be of three types (Case):
1. latent infection with only serological but no clinical evidence;
2. acute or subacute brucellosis
3. chronic brucellosis.

11. Diagnosis
1. Blood culture is the most definitive method for the diagnosis of
brucellosis.
• Blood is inoculated into a bottle of trypticase soy broth or brucella broth in
a biphasic blood culture bottle, also called the Castaneda method.
• It is incubated at 37°C under 5- 10% CO2•
• Ausually scanty, large volumes of blood (5 ml) should be inoculated.
Subcultures are made on solid media every 3-5 days, beginning on the
fourth day.
• Growth may often be delayed and cultures should not be declared negative
in less than 6-8 weeks.
• Automated cultures may become positive in 5-6 days.

12. • The standard agglutination test (SAT) is performed most often. This is
a tube agglutination test in which equal volumes of serial dilutions of
the patient's serum and the standardised antigen (a killed suspension
of a standard strain of B.abortus) are mixed and incubated at 37°C for
24 hours or 50°C for 18 hours. A titre of 160 or more is considered
significant.
• Most patients with acute brucellosis develop titres of 640 or more by
3-4 weeks of illness.
• Titres tend to decline after the acute phase of the illness.

13. • For the detection of infected animals in dairies, pooled milk samples may be
tested for bacilli by culture and for antibodies by several techniques.
• In the milk ring test, a sample of whole milk is mixed well with a drop of the
stained brucella antigen (a concentrated suspension of killed B.abortus stained
with hematoxylin) and incubated in a water bath at 70°C for 40-50 minutes.
• If antibodies are present in the milk, the bacilli are agglutinated and rise with
the cream to form a blue ring at the top, leaving the milk unstained. If
antibodies are absent, no coloured ring is formed and the milk remains
uniformly blue.
• The whey agglutination test is another useful method for detecting antibodies
in milk.
• Rose Bengal card test and rapid plate agglutination tests can also be used for
screening infected herds.

14. Treatment
• The usual regimen for adults is a combination of doxycycline for 45
days with streptomycin IM daily for the first two weeks, and for
children, cotrimoxazole with rifampicin or gentamycin.