The genus Staphylococcus encompasses Gram-positive bacteria found on the skin and mucous membranes of people and warm-blooded animals. These organisms appear spherical and characteristically form irregular, grape-like clusters. While many species exist harmlessly as part of the normal human microbiota, a few have the potential to cause a wide spectrum of diseases, ranging from minor skin conditions to severe, life-threatening systemic illnesses.
Identifying the Key Species
The medical relevance of Staphylococcus species is determined by their ability to produce the enzyme coagulase, which clots blood plasma. This enzyme divides the species into two major groups: Coagulase-Positive and Coagulase-Negative Staphylococci (CoNS). The most medically significant species, Staphylococcus aureus, is coagulase-positive, indicating a higher degree of virulence. S. aureus is frequently carried in the anterior nares, or nose, of approximately 30% of healthy adults.
Coagulase-Negative Staphylococci (CoNS) include Staphylococcus epidermidis and Staphylococcus saprophyticus. S. epidermidis is an abundant resident of the human skin, where it usually remains harmless. However, its ability to form a protective biofilm on foreign materials makes it a common cause of infections related to medical devices. S. saprophyticus is notable primarily for causing urinary tract infections, particularly in young women.
Common Staphylococcal Infections
Staphylococcus aureus is responsible for the majority of human staphylococcal diseases, often through direct tissue invasion or the release of powerful toxins. The most common manifestations are localized skin and soft-tissue infections, which begin when the bacteria enter the body through a cut or abrasion. These localized infections include impetigo (crusting sores) and boils (furuncles) or carbuncles (deeper abscesses).
Infections become more serious if the bacteria invade deeper tissues or enter the bloodstream, leading to systemic illnesses. Once in the bloodstream (bacteremia), S. aureus can spread to infect various organs, causing pneumonia, bone infections (osteomyelitis), or endocarditis. Some S. aureus strains produce potent exotoxins that cause systemic diseases without widespread bacterial invasion. These toxin-mediated illnesses include staphylococcal food poisoning and toxic shock syndrome.
Coagulase-Negative Staphylococci are significant pathogens, especially in healthcare settings. S. epidermidis causes many hospital-acquired infections, often associated with indwelling medical devices like catheters, artificial heart valves, or prosthetic joints. The bacteria adhere to the foreign material and secrete a sticky biofilm that shields them from antibiotics and the host’s immune defenses. These infections can be chronic and often require the removal of the contaminated device for successful treatment.
Understanding Antibiotic Resistance
The frequent use of antibiotics has driven the evolution of drug-resistant strains within the Staphylococcus genus. The most recognized example is Methicillin-Resistant S. aureus (MRSA), which is resistant to methicillin, penicillin, and the entire class of beta-lactam antibiotics. MRSA resistance is mediated by the acquisition of the staphylococcal chromosomal cassette mec (SCCmec), which carries the mecA gene. This gene directs the bacteria to produce the altered protein PBP2a, which performs cell wall construction but is not targeted by beta-lactam drugs.
The difficulty in treating MRSA led to the increased reliance on vancomycin, a powerful antibiotic for severe infections. However, S. aureus has evolved strains that exhibit reduced susceptibility to vancomycin, known as Vancomycin-Intermediate S. aureus (VISA), and fully resistant strains, Vancomycin-Resistant S. aureus (VRSA). VRSA strains acquire high-level resistance by incorporating the vanA gene, often transferred from other resistant bacteria like Enterococcus. This genetic transfer allows the bacteria to alter their cell wall structure, preventing vancomycin from binding effectively and limiting treatment options.
Prevention and Management
Preventing the spread of staphylococcal infections relies heavily on maintaining rigorous personal hygiene.
Prevention Methods
Consistent handwashing with soap and water for at least 20 seconds is recommended.
Use alcohol-based hand sanitizers.
Keep all cuts, scrapes, and abrasions clean and covered until they are fully healed.
Avoid sharing personal items like towels, razors, and athletic gear.
Treatment for staphylococcal infections begins with determining if the strain is susceptible to standard antibiotics. Minor skin infections can often be managed with topical antibiotics or by surgically draining abscesses or boils. For systemic infections caused by non-resistant strains, conventional antibiotics are used, often intravenously in severe cases. Infections caused by resistant strains like MRSA require the use of specialized antibiotics and may involve a longer course of therapy.