BRUCELLA MELITENSIS: CHARACTERISTICS, MORPHOLOGY, PATHOLOGIES - BIOLOGY - 2025

Brucella melitensis is a Gram negative coccobacillary bacterium that produces a zoonotic disease called ovine and caprine brucellosis. The infection causes significant economic losses by causing abortions in sheep and goats.

Despite the fact that this disease had been eradicated from some nations, today B. melitensis is considered to be a re-emerging pathogen, especially in the Middle East.

Brucella melitensis colonies on Brucella melitensis blood / Gram agar

Also this disease is prevalent in the Mediterranean, Central Asia, Africa, India, Arabian Gulf and some countries in Central America and areas of Mexico.

Humans can be infected tangentially with this bacterium, mainly occupationally exposed people, that is, those who handle infected animals. People can also get sick from eating contaminated dairy products.

It should be noted that of all the species of the genus Brucella, the melitensis species is the most virulent. Its pathogenic power makes it a bacterium with great potential to be used in bioterrorist attacks.

characteristics

Phylum: Proteobacteria

Class: Alphaproteobacteria

Order: Rhizobiales

Family: Brucellaceae

Genus: Brucella

Species: melitensis

Morphology

They are Gram negative coccobacilli or short rods 0.5–0.7 µm in diameter by 0.6–1.5 µm long. They are non-sporulated, non-capsulated, non-mobile, and facultative aerobic.

They have the ability to live intracellularly within the body and multiply in culture media extracellularly. They are distributed in isolation, in pairs or in groups.

The colonies are small, convex, smooth, translucent, slightly yellowish, and opalescent, and may turn brown with age.

Virulence factors

Strains of the genus Brucella in the laboratory initially form smooth colonies, and as subcultures are carried out, they suffer antigenic variations and become rough colonies.

The microorganisms at the time of producing smooth colonies are resistant to intracellular destruction by polymorphonuclear cells, that is, smooth cells are more virulent than rough ones.

On the other hand, this microorganism has two main antigenic determinants, called A and M.

Transmission

Direct human contact with placentas, fetuses, fetal fluids, and vaginal fluids from infected animals are the primary source of contagion. As well as by ingestion of contaminated food (meat, milk, dairy in general) or by inhalation of aerosolized microorganisms.

Sheep continue to shed the organism in their vaginal fluids for three weeks after calving or abortion. Meanwhile, goats can last 2 to 3 months releasing the bacteria in the same fluids.

Infected animals also excrete the organism for a long time and sometimes permanently through colostrum, milk and semen. Likewise, pups that nurse from infected mothers can shed the bacteria in their feces.

This means that animals can be infected both horizontally (through close contact with each other) and vertically (mother to child).

The digestive system, the oropharyngeal mucous membrane, the respiratory tract, the conjunctiva, and skin wounds serve as a gateway.

In exceptional cases by sexual contact, since contagion by semen occurs mainly in artificial inseminations of animals.

The microorganism can also be spread in nature through inanimate objects, as well as by mechanical transport through carnivorous animals, which can carry contaminated material when hunting infected goats or sheep.

Pathogenesis in animals

Once the microorganism enters the body by any route, they are phagocytosed by cells of the immune system. Not all bacteria survive, but if they are not destroyed they multiply in the endoplasmic reticulum.

They are disseminated by the hematic route, showing a predilection for the animal's reproductive system and the mammary glands. They multiply abundantly in the placental cotyledons and chorion, as well as in fetal fluids, causing injury to the organ wall.

This causes an ulcerative endometriosis in the intercotyledonary spaces and destruction of the villi that cause the death and expulsion of the fetus.

Pathology and clinical manifestations in animals

Brucella melitensis infection in non-pregnant goats and sheep can be asymptomatic. In pregnant females it produces abortions, fetal deaths, premature births and weak offspring.

In males it can cause epididymitis, acute orchitis and prostatitis that can lead to infertility of the animal. Arthritis can also be observed, although infrequently, in both sexes.

Infection with B. melitensis in other unusual animal species can cause the same symptoms.

At necropsy, granulomatous inflammatory lesions are seen in the reproductive tract, udder, supramammary lymph nodes, joints, synovial membranes, and other lymphoid tissues.

Placentitis with edema, necrosis of the cotyledons, and a rough and thickened appearance of the intercotyledonary space can be seen.

The fetus can be seen as normal, autolysed, or with spots of blood and excess fluid.

Pathogenesis in humans

The microorganisms enter through the digestive tract, through the skin or mucosa, there they are phagocytosed, being able to survive inside the cell, inactivating the myeloperoxidase-peroxide system.

From there they are transported to the lymph nodes and bacteremia occurs. Subsequently, there is sequestration of the microorganisms in various organs of the reticuloendothelial system (liver, spleen, bone marrow).

By degenerating the PMN, it releases the microorganism, which is then endocyted by another cell and this cycle repeats itself.

This explains the episodes of undulating fever, associated with the release of bacteria and some bacterial components, such as lipopolysaccharides (LPS).

The release of the bacteria to the peripheral circulation favors the hematogenous seeding of other organs and tissues. Ultimately, the pathological spectrum will depend on:

• The immune status of the host,
• Presence of underlying diseases and
• The species responsible for the infection, remembering that melitensis is the most virulent of all species.

Pathology and clinical manifestations in humans

Brucellosis in humans is known by various names, including: undulating fever, Bang's disease, Gibraltar fever, Mediterranean fever, and Malta fever.

The onset of symptoms can be insidious or abrupt. Nonspecific symptoms are fever, night sweats, chills and malaise, severe headache, myalgia, and arthralgia.

These symptoms may be accompanied by lymphadenopathy, splenomegaly, and hepatomegaly. Erythema nodosum-like skin lesions and maculopapular or papulonodular eruptions may sometimes occur.

Undulating fever owes its name to the periodic appearance of it. This fever is generally nocturnal and lasts for weeks, months and years with periods between afebrile periods, repeating the cycles. Therefore it becomes a chronic and debilitating disease.

Among the most severe complications that could occur are: chronic fatigue, endocarditis, blood vessel thrombosis, epididymis-orchitis and nephritis. At the neurological level: meningitis, cerebral hemorrhages, encephalitis, uveitis and optic neuritis.

On the respiratory system can be seen: interstitial pneumonitis, empyema and pleural effusion. In the gastrointestinal and hepatobiliary systems: colitis, enterocolithia or spontaneous peritonitis, caseous hepatic granulomas and microabscesses, and splenic abscesses.

At the osteoarticular level: arthritis (bursitis, sacroiliitis, spondylitis and osteomyelitis).

Diagnosis

The ideal samples to isolate the microorganism in humans are blood and bone marrow samples, tissue biopsies and CSF can also be used.

The organism grows very slowly in blood culture bottles incubated at 35 ° C for 4 to 6 weeks, with periodic subcultures on blood and chocolate agar. BACTEC systems can detect growth after 7 days of incubation.

Brucella melitensis does not produce hydrogen sulfide, it does not require CO ₂ for its growth, it is catalase and oxidase positive. It grows in the presence of the following dyes: basic fuchsin 20 µg, Thionin (20 and 40 µg) and Blue Thionin 2 µg / mL.

They can be smeared from isolated colonies and stained with modified Ziehl-Neelsen, using weak acids. Although B. melitensis is not a properly acid-alcohol resistant bacteria, under this modified technique it will stain red.

Finally, the agglutination technique with specific antisera can be used for diagnosis.

Prevention

The disease in animals is preventable by the application of the vaccine and the sacrifice of animals with serological signs of infection.

It should be ensured that the females stop in open and dry spaces, since the closed, humid and dark spaces favor the proliferation of the bacteria. Likewise, pregnant females should be separated from the rest of the group. It is also convenient to disinfect the farrowing pens, remove fetuses, placenta and any infected material.

In humans, it is prevented by avoiding the consumption of unpasteurized milk and dairy products without sanitary control.

Veterinarians, animal caretakers, among other occupationally exposed persons, must take protective measures when handling animals, the environment and their biological fluids.

Bioanalysts and microbiologists must work the cultures under a biological safety cabinet, complying with the rules for handling microorganisms of Level 3 of biosafety.

They should also avoid procedures associated with the emission of aerosols: aspiration of liquids with a syringe, centrifugation of contaminated material, energetic pipetting, among others.

Treatment

The animals are not treated, they are sacrificed.

In humans, a combination of tetracycline with an aminoglycoside or also trimethoprim-sulfamethoxazole can be used.

Despite treatment and remission of symptoms, there may be recurrences.

References

1. Koneman E, Allen S, Janda W, Schreckenberger P, Winn W. (2004). Microbiological Diagnosis. (5th ed.). Argentina, Editorial Panamericana SA
2. Forbes B, Sahm D, Weissfeld A. Bailey & Scott Microbiological Diagnosis. 12 ed. Argentina. Editorial Panamericana SA; 2009.
3. González M, González N. Manual of Medical Microbiology. 2nd edition, Venezuela: Directorate of media and publications of the University of Carabobo; 2011
4. The Center for Food Security & Public Health. Iowa State University. Brucellosis Ovine and Caprine: Brucella melitensis. 2009, pp 1-5
5. SAG Ministry of Agriculture Chile: Technical file. Caprine brucellosis and Ovine (Brucella melitensis).
6. Díaz E. Epidemiology of Brucella melitensis, Brucella suis and Brucella abortus in domestic animals. Rev. Sci. Tech. Off. Int. Epiz, 2013; 32 (1): 43-51
7. Ruiz M. Manual for the epidemiological surveillance of Brucellosis. Secretary of Health, United Mexican States. Pp. 1-48
8. Female CV, Wagner MA, Eschenbrenner M, Horn T, Kraycer JA, Redkar R, Hagius S, Elzer P, Delvecchio VG. Global analysis of Brucella melitensis proteomes. Ann NY Acad Sci. 2002; 969: 97-101.