The Gram-positive bacterium Staphylococcus hominis is a species frequently isolated in clinical microbiology laboratories. This organism naturally lives on human skin, yet it has the potential to cause serious disease. When a lab test returns positive for S. hominis, the finding must be carefully evaluated to determine if it represents a harmless result of sample collection or a true, spreading infection. Distinguishing between contamination and a significant health threat is an important step in patient care.
Classification and Role as Skin Flora
Staphylococcus hominis belongs to a large grouping of microbes known as Coagulase-Negative Staphylococci (CoNS), named because they do not produce the enzyme coagulase. This bacterium is a ubiquitous and normal inhabitant of the human skin, residing there without causing harm in healthy individuals. It is considered one of the most abundant CoNS species found, second only to Staphylococcus epidermidis.
The natural habitat of S. hominis is concentrated in areas with numerous apocrine glands, such as the axillae and pubic regions. Strains of this organism colonize the skin surface, typically persisting for weeks to months at a time. Because it is so widespread on the body, it is easily introduced into collected specimens during the blood draw or sample collection process. This biological reality is the primary reason S. hominis frequently appears in cultures as a contaminant.
Interpreting Lab Results: Contaminant or True Infection?
Determining the clinical significance of S. hominis isolation is the greatest diagnostic challenge. When the organism is recovered from a blood culture, the clinical team must analyze multiple factors to classify the result as contamination or a true infection. Contamination is strongly suspected when only a single blood culture bottle yields a positive result. In these cases, the organism was likely picked up from the patient’s skin during the venipuncture procedure, despite antiseptic skin preparation.
A true infection is suggested when multiple sets of blood cultures, drawn from different sites or at different times, all grow the same organism. This finding significantly lowers the probability that the organism is a simple skin contaminant. Clinicians also integrate the laboratory data with the patient’s clinical status. A positive culture accompanied by systemic signs of infection, such as fever, an elevated white blood cell count, or low blood pressure, strongly suggests a genuine bloodstream infection. Furthermore, the method of sample collection can be telling; a positive culture from a sample drawn through a central venous catheter, compared to a peripheral vein draw, may increase suspicion for a device-related infection.
Clinical Manifestations of S. hominis Infection
While typically a harmless colonizer, S. hominis acts as an opportunistic pathogen, causing infection almost exclusively in specific patient populations. The risk of disease increases substantially in individuals who are immunocompromised, such as those undergoing chemotherapy or with underlying severe illnesses. A history of recent hospitalization or the presence of implanted medical devices are also significant predisposing factors.
The organism is one of the most common causes of hospital-acquired (nosocomial) bloodstream infections. It frequently causes catheter-related bloodstream infections (CRBSIs) because of its ability to form a protective matrix known as a biofilm on foreign materials. Once established on a device, the biofilm shields the bacteria from both the body’s immune response and antibiotic treatment. In less frequent cases, S. hominis can cause endocarditis, an infection of the heart’s inner lining or valves, particularly prosthetic ones.
Treatment Approaches and Antibiotic Resistance
The therapeutic approach for a confirmed S. hominis infection is complicated by its high rate of resistance to common antibiotics. A large percentage of S. hominis isolates are resistant to methicillin and other penicillin-related drugs, referred to as Methicillin-Resistant Coagulase-Negative Staphylococci. This resistance is often mediated by the acquisition of the mecA gene, which makes many beta-lactam antibiotics ineffective.
Because of this common resistance pattern, initial treatment for a serious infection typically begins with intravenous Vancomycin until susceptibility testing results are available. Alternative effective agents for methicillin-resistant strains include Linezolid or Daptomycin. If the isolate is susceptible to methicillin, a switch to Oxacillin or Cefazolin may be recommended. For infections associated with medical devices, the infected foreign material usually must be removed or replaced in addition to the course of antibiotics to achieve a cure.