The genus Staphylococcus refers to a group of bacteria commonly found living on the skin and in the nose of humans and animals. These bacteria are spherical, Gram-positive organisms that often appear in grape-like clusters under a microscope. While an estimated 30% of healthy adults carry Staphylococcus aureus in their nose, this colonization is often harmless and asymptomatic. Problems arise if the bacteria breach the skin barrier and become pathogenic, causing infection. The resulting infections can range from minor skin irritations to severe, life-threatening systemic diseases.
The Two Major Categories of Staph
Microbiologists traditionally divide Staphylococcus species into two broad clinical categories based on their ability to produce the enzyme coagulase, which causes blood plasma to clot. The coagulase test helps differentiate the most medically significant staph species from the others. Coagulase-Positive Staphylococci (CoPS) primarily consist of Staphylococcus aureus, which is the most dangerous and highly pathogenic species.
Coagulase-Negative Staphylococci (CoNS) include over 30 other species, such as S. epidermidis and S. saprophyticus. These species are generally considered less virulent and are common commensals of the skin and mucous membranes. CoNS are increasingly recognized as major pathogens in hospital settings, especially when they colonize medical devices like catheters or artificial joints, where they can form protective biofilms. They present a growing challenge in healthcare due to their prevalence and propensity for antibiotic resistance.
Common Sites of Staph Infections
When staph bacteria, particularly S. aureus, gain entry into the body through a break in the skin, the resulting infections are broadly classified as either localized or invasive. Localized infections are the most common presentation and typically involve the skin and soft tissue, often resembling a pimple or a spider bite. Examples include folliculitis (an infection of the hair follicle), and boils (furuncles) or carbuncles, which are painful abscesses filled with pus.
The infection can also spread to deeper layers of skin and tissue, causing cellulitis, which presents as a swollen, hot, and red area. Invasive infections occur when the bacteria travel from the initial site into the bloodstream. Once in the blood, the bacteria can cause bacteremia, which may lead to sepsis, a potentially fatal immune response.
Invasive staph infections can target distant organs, resulting in severe conditions. These include pneumonia and endocarditis, an infection of the heart valves. The bacteria can also infect bone tissue (osteomyelitis) or cause septic arthritis, affecting joints like the knee or hip. These invasive forms of infection require aggressive treatment compared to a superficial skin lesion.
Understanding Antibiotic Resistance
The term “strains” is most frequently used in a public health context to distinguish between staph bacteria that are sensitive to common antibiotics and those that are resistant. Methicillin-Sensitive S. aureus (MSSA) is susceptible to beta-lactam antibiotics, such as methicillin, penicillin, and oxacillin. These are the standard treatments for staph infections acquired outside of the hospital. However, the most concerning strain is Methicillin-Resistant S. aureus (MRSA), which has acquired a gene that makes it immune to this entire class of drugs.
MRSA strains possess the mecA gene, which allows the bacteria to produce a modified cell wall protein called PBP2a. This protein allows the bacteria to build their cell walls even when beta-lactam antibiotics are present, rendering the drugs ineffective. MRSA is typically resistant to many other commonly used antibiotics, making infections difficult to treat.
MRSA is further categorized by where it is acquired, primarily as Healthcare-Associated MRSA (HA-MRSA) and Community-Associated MRSA (CA-MRSA).
Healthcare-Associated MRSA (HA-MRSA)
HA-MRSA strains are linked to healthcare facilities, where they affect patients with weakened immune systems, indwelling devices, or surgical wounds. These strains often carry genetic elements that confer resistance to multiple non-beta-lactam antibiotics.
Community-Associated MRSA (CA-MRSA)
CA-MRSA emerged in the community among healthy individuals who had no recent contact with a healthcare setting. These community strains often cause skin and soft-tissue infections and are genetically distinct from HA-MRSA. While CA-MRSA is often susceptible to more non-beta-lactam antibiotics than HA-MRSA, its aggressive nature and ability to spread rapidly in the community pose a unique public health challenge.
Prevention and Controlling Spread
Controlling the spread of staph, especially resistant strains like MRSA, relies heavily on consistent hygiene practices. Since staph bacteria are commonly transmitted through direct skin-to-skin contact or contaminated objects, frequent handwashing is the most effective preventive measure. Hands should be washed thoroughly with soap and water or cleaned with an alcohol-based hand sanitizer.
Proper wound care is also necessary to prevent bacteria from entering the body or spreading from an infected site. Any cuts, scrapes, or skin lesions should be washed immediately and kept covered with a clean, dry bandage until they are fully healed. It is important to avoid touching or picking at sores, as this can easily transfer the bacteria.
Preventing transmission involves avoiding the sharing of personal items that contact the skin. These items can harbor the bacteria for hours or weeks and should not be shared. Examples include:
- Towels
- Washcloths
- Razors
- Clothing
- Athletic equipment
Regular cleaning and disinfection of frequently touched surfaces, especially in communal environments, further minimizes the risk of environmental contamination.