The genus Corynebacterium comprises a large group of bacteria, many of which are commonly found on skin and mucous membranes. Corynebacterium bovis has traditionally been studied for its significance in veterinary medicine, particularly within livestock populations. While long associated with cattle, recent clinical reports show C. bovis is increasingly recognized as an emerging cause of opportunistic infection in humans. This shift necessitates a clearer understanding of its specific characteristics, how it crosses the species barrier, and the proper medical management of the resulting infections.
Defining Corynebacterium bovis
Corynebacterium bovis is a Gram-positive bacterium characterized microscopically as a small, non-spore-forming rod, often appearing club-shaped or coccobacillary. It is facultatively anaerobic, meaning it can survive with or without oxygen. A distinguishing feature of this species is its lipophilic nature, requiring or being stimulated by lipids for optimal growth.
The bacterium’s primary ecological niche is the skin and mammary gland of cattle, where it often exists as a commensal organism. In veterinary practice, C. bovis is a well-known agent of bovine mastitis, a contagious inflammation of the udder. This infection is typically subclinical but can lead to economic losses for dairy farmers due to reduced milk production. The organism is also a common opportunistic pathogen in research colonies of immunodeficient animals, such as nude mice, where it causes a characteristic scaly skin disease.
Zoonotic Transmission and Human Exposure Routes
Corynebacterium bovis is classified as a zoonotic pathogen that transmits from animals to humans, with livestock being the main reservoir. Transmission frequently occurs through direct contact with infected animals or their contaminated environments. Individuals with close occupational exposure, such as veterinarians, dairy farmers, and farmhands, face the highest risk of acquiring the organism.
The mechanism of transmission often involves breaching the human skin barrier, allowing the bacteria to enter the body. Exposure to contaminated milk or dairy products is a plausible route, as the bacterium colonizes the bovine udder. Its lipophilic nature allows it to persist in lipid-rich environments, such as animal skin flakes, making environmental contamination a significant factor in its spread.
Aerosolization has been observed in laboratory settings involving infected rodent colonies, suggesting airborne dispersal is a viable route in confined spaces. Non-traditional exposure has also been noted in clinical settings, such as surgical site infections hypothesized to stem from contaminated cryopreserved fat. These varied pathways show that human infection can occur through direct contact, environmental persistence, or iatrogenic introduction in healthcare.
Clinical Profiles of Human Infection
In humans, C. bovis acts predominantly as an opportunistic pathogen. The majority of reported infections occur in people with underlying health conditions or compromised immune systems, such as those with cancer, HIV, or those receiving immunosuppressive therapies.
The spectrum of clinical presentations is broad, often involving the bloodstream or sites associated with medical devices. The organism frequently causes bacteremia, a bloodstream infection often related to central venous catheters or other indwelling lines. One of the most serious infections is endocarditis, an inflammation of the heart’s inner lining or valves.
C. bovis has also been implicated in infections where it forms biofilms on foreign material. These include:
- Infections of prosthetic joints and shunts.
- Infections involving the central nervous system, such as meningitis and brain abscesses (though these are rare).
- Localized presentations, including chronic otitis media, persistent skin ulcers, and keratitis.
Diagnostic Identification and Treatment Protocols
Diagnosing a C. bovis infection presents a challenge because the organism is easily mistaken for a harmless skin commensal. It belongs to the group of non-diphtheria corynebacteria often dismissed as contaminants when isolated from culture. Accurate identification requires advanced laboratory techniques to distinguish C. bovis from other species.
Specialized methods like Matrix-Assisted Laser Desorption/Ionization-Time of Flight mass spectrometry (MALDI-TOF MS) and 16S ribosomal RNA gene sequencing are commonly used for definitive species-level identification. Finding C. bovis in multiple deep-tissue or blood cultures strongly indicates it is a true pathogen rather than a contaminant. Because of its lipophilic nature, specialized culture media containing lipids may be necessary to support its growth.
Antimicrobial management requires careful consideration of the organism’s variable resistance patterns, which can include resistance to common antibiotics like penicillin and other beta-lactams. For serious systemic infections, treatment relies heavily on susceptibility testing. Glycopeptide antibiotics, such as vancomycin or teicoplanin, are typically the initial drugs of choice for severe infections due to their reliable activity.
Treatment often requires prolonged antibiotic courses, especially for endocarditis. A primary component of managing device-related infections (catheters, shunts, or prosthetic joints) is the physical removal of the infected foreign body. The bacteria’s ability to form biofilms makes antibiotic therapy alone frequently unsuccessful unless the colonized material is fully extracted. Combination therapy with a second agent, like rifampin, may be used to enhance clearance in deep-seated infections.