Brucellosis is an infectious disease caused by bacteria. People can get the disease when they are in contact with infected animals or animal products contaminated with the bacteria. Animals that are most commonly infected include sheep, cattle, goats, pigs, and dogs, among others. Brucellosis Worldwide.

1. Brucella species

2. Brucella species
• Brucella species are small (0.6 × 0.6 to 1.5 μm),
non-motile, coccobacillary, Gram-negative
bacteria.
• They are not decolorized by 0.5% acetic acid in the
modified Ziehl-Neelsen (MZN).
• Smears of body fluids or
tissues, they appear as
clusters of red coccobacilli.

3. • All species of Brucella share a high degree of
genetic similarity.
• Brucella melitensis, B. abortus and B. suis are
subdivided into biovars based on cultural and
serological properties.
• Brucella genome is unusual it composed of
two circular chromosomes, with the exception
of B. suis biovar 3, which has a single
chromosome.

4. • Brucella ovis and some biotypes of B.
abortus require 5 to 10% CO2 for primary
isolation.
• Moreover, the growth of other Brucella
species is enhanced in an atmosphere of
CO2.
• Media enriched with blood or serum are
required for culturing B. abortus biotype 2
and B. ovis.

6. Usual habitat
• Brucellae have a predilection for both female and
male reproductive organs in sexually mature
animals and each Brucella species tends to infect a
particular animal species.
• Infected animals serve as reservoirs of infection,
which often persists indefinitely.
• Organisms shed by infected animals can remain
viable in a moist environment for many months.
• Transmission is usually through direct contact with
infected animals or fluids and tissues associated
with abortion.

7. Differentiation of Brucella species
• Brucella species are differentiated by colonial
appearance, biochemical tests, specific cultural
requirements and growth inhibition by dyes.
• In addition, agglutination with monospecific sera,
susceptibility to bacteriophages and molecular
methods are employed for definitive
identification.

8. • On primary isolation, colonies of B. abortus, B.
melitensis and B. suis occur in smooth forms and
are small, glistening, bluish and translucent after
incubation for 3 to 5 days. Colonies become
opaque with age.
• In contrast, primary isolates of B. ovis and B. canis
always occur in rough forms. These rough colonies
are dull, yellowish, opaque and friable.
• Brucellae are non-haemolytic on blood agar.

9. • Slide agglutination tests with
monospecific antisera are used to detect
the presence of important surface
antigens, abortus antigen A and
melitensis antigen M.
• The R antigen, a feature of the rough
brucellae B. ovis and B. canis, can be
detected by anti-R serum.

10. • Isolates of B. abortus are lysed by a specific
bacteri-ophage (Tbilisi phage) at routine test
dilution.
• PCR, PCR restriction fragment length
polymorphism, pulsed-field gel electrophoresis
methods and other molecular methods have been
developed for identification and differentiation of
Brucella isolates.

11. Pathogenesis and pathogenicity
• The establishment and outcome of infection with
brucellae depend on the number of infecting
organisms and their virulence and also on host
susceptibility, including age of the host.
• Brucellae which lack the major outer-membrane
lipopolysaccharide, produce rough colonies and
are less virulent than those derived from smooth
colonies.

12. • Smooth and rough organisms can enter host cells,
rough forms are usually eliminated unlike smooth
forms which may persist and multiply.
• Brucellae persist within macrophages but not
within neutrophils.
• Non-opsonized brucellae are taken up through
interaction of the O side-chains of LPS with
cholesterol-rich regions of the phagocyte plasma
membrane, termed lipid rafts.

13. • Once engulfed, brucellae persist within the
acidified phagosome, or ‘Brucella-containing
vacuole’.
• Acidification of the phagosome is important as it
induces changes in gene expression of the brucella
organism, which favour intracellular survival.
• Cyclic beta-1,2-glucans, constituents of the outer
membrane, help in the prevention of
phagolysosome fusion.
• Inhibition of phagosome–lysosome function is a
major mechanism for intracellular survival and an
important determinant of bacterial virulence.

14. -While survival within the phagosome occurs,
replication of brucellae only takes place once the
‘brucellosome’ is formed.
This structure is formed through the fusion of the
Brucella-containing vacuole with the
rough endoplasmic reticulum of the host cell.
-Effectors secreted by a type IV secretion system
encoded by the virB operon appear to be important
in maturation of the vacuole and in its transport to,
and fusion with, the rough endoplasmic reticulum

15. -In the next phase of infection, virulent brucellae
are transported to regional lymph nodes.
Intermittent bacteremia results in spread and
localization in the reproductive organs and
associated glands in sexually mature animals.
Erythritol, a polyhydric alcohol which acts as a
growth factor for brucellae, is present in high
concentrations in the placentae of cattle, sheep,
goats and pigs.

16. -This growth factor is also found in other organs
such as the mammary gland and epididymis,
which are targets for brucellae.
-Intracellular replication in trophoblastic cells is
strongly influenced by the stage of gestation and
increases in late gestation, when the cells
actively secrete steroid hormones.
- In chronic brucellosis, organisms may localize
in joints or intervertebral discs.

17. It is suggested that brucellae may inhibit or delay
the host immune response and this may be
responsible in part for the persistent infections
seen with this pathogen.

18. Table 33.1 Brucella species, their host range and the
clinical significance of infection.
Brucella
species
Usual host / Clinical
significance
Species occasionally infected
/ Clinical significance
B. abortus Cattle / Abortion, orchitis Sheep, goats, pigs / Sporadic
abortion
Horses / Bursitis
Humans / Intermittent fever,
systemic disease
B. melitensis Goats, sheep / Abortion,
orchitis, arthritis
Cattle / Sporadic abortion,
brucellae in milk
Humans / Malta fever, severe
systemic disease
B. ovis Sheep / Epididymitis in rams,
sporadic abortion in ewes