Staphylococcus hominis is a Coagulase-negative Staphylococcus (CoNS) commonly residing on human skin as a commensal organism. While generally harmless, S. hominis can become an opportunistic pathogen, causing infection when the host’s defenses are weakened. Its ability to cause serious illness is typically limited to specific circumstances. For healthy individuals, this bacterium rarely leads to medical concern, but in a hospital setting, it requires immediate attention.
Where S. hominis Causes Infection
Staphylococcus hominis is recognized as a significant cause of hospital-acquired, or nosocomial, infections, especially in patients with weakened immune systems. The bacterium gains entry into the body when the natural skin barrier is compromised, such as during surgical procedures or through the insertion of medical devices. It is particularly known for its ability to form a dense, protective layer called a biofilm on foreign materials inside the body.
This biofilm formation is a major factor in infections related to indwelling medical devices, including prosthetic joints, heart valves, and central venous catheters. Once a biofilm forms on a catheter, the bacteria are shielded from both the immune system and many antibiotics, frequently leading to bloodstream infections, or bacteremia. A bloodstream infection caused by S. hominis can be life-threatening and is a common clinical presentation, often necessitating aggressive treatment and device removal.
Confirming the Infection: Laboratory Testing
Diagnosing an active S. hominis infection requires careful laboratory work. The process begins with collecting specimens from the suspected site of infection, such as blood, pus, or the tip of a removed catheter. These clinical samples are then placed onto specialized growth media, like blood agar, to culture and isolate the bacteria.
The initial identification involves confirming the organism is a Gram-positive coccus that clusters together, followed by biochemical tests to classify it as a Coagulase-negative Staphylococcus. Precise species identification may rely on automated systems or molecular methods, such as Polymerase Chain Reaction (PCR), to detect specific genetic markers. Antimicrobial Susceptibility Testing (AST) determines how the isolated strain reacts to a panel of different antibiotics. AST is performed using methods like the Kirby-Bauer disk diffusion test or by determining the Minimum Inhibitory Concentration (MIC) of various drugs.
Initial and Standard Treatment Protocols
Treatment often begins with empirical therapy, which is started before the results of the AST are available. Because of the high probability of resistance in hospital settings, the initial choice for serious infections is often intravenous vancomycin. This broad-spectrum antibiotic is selected to provide immediate coverage against methicillin-resistant strains. The goal of this initial treatment is to stabilize the patient while awaiting the susceptibility report.
Once the AST results are finalized, the treatment is adjusted to target the specific strain, a process known as definitive therapy. If the isolated S. hominis strain is found to be susceptible to beta-lactam antibiotics, treatment can be switched to drugs like oxacillin or cefazolin. These traditional cell wall-targeting antibiotics are highly effective against susceptible strains and may be preferred over vancomycin due to lower toxicity risks. The duration of therapy depends heavily on the infection site and severity; uncomplicated bacteremia may require two weeks of treatment, whereas complicated infections like endocarditis may need six weeks or more.
The Challenge of Antibiotic Resistance
Multi-drug resistance is frequently found in S. hominis infections. S. hominis strains often carry the mecA gene, which confers resistance to methicillin and all related beta-lactam antibiotics. This resistance means that penicillins and cephalosporins are ineffective, forcing clinicians to rely on a different class of drugs.
When resistance is confirmed, vancomycin is the first-line treatment for serious bloodstream infections. However, some strains of S. hominis have shown reduced susceptibility to vancomycin, prompting the use of alternative agents. These alternatives include linezolid and daptomycin, which are reserved for resistant or complicated cases. Daptomycin, a lipopeptide antibiotic, has shown high efficacy against many Coagulase-negative Staphylococci, and resistance to it is currently rare. Linezolid, an oxazolidinone, is another effective option, although resistance has been reported and it is sometimes avoided in combination with other drugs due to potential antagonistic effects.