Staphylococcus Species and Gram-Positive Cocci Microbiology

General Classification of Gram-Positive Cocci

Gram-positive cocci that grow in aerobic conditions are primarily categorized into several genera based on their biochemical characteristics, specifically the catalase test. The genus Staphylococcus is characterized as being catalase-positive, which distinguishes it from the catalase-negative genera Streptococcus and Enterococcus. Other relevant genera in this category include Micrococcus. This material is provided by Alicja Ekiel from the Department and Clinic of Medical Microbiology, Faculty of Medical Sciences (WNM) in Katowice, Silesian Medical University (SUM). Students are instructed to supplement these notes with chapter 6.1.1, "Rodzaj Staphylococcus," in the textbook "Mikrobiologia lekarska" by M. Bulanda, A. Pietrzyk, and M. Wróblewska (PZWL).

The Genus Staphylococcus: Characteristics and Morphology

The name Staphylococcus originates from the Greek words "staphyle," meaning a cluster of grapes, and "coccus," meaning a grain or seed. These microorganisms are Gram-positive cocci that typically form grape-like clusters under microscopic examination. They are catalase-positive, do not produce spores, and are non-motile. Their metabolism is either aerobic or facultatively anaerobic. A defining physiological trait is that they are halophytes, meaning they can tolerate high salt concentrations, specifically up to $10\%\,NaCl$ in their growth medium. Staphylococci are part of the normal microbiota, occurring naturally on the skin and mucous membranes of humans, other mammals, and birds.

Two major diagnostic indicators on culture media are Beta-haemolysis (the complete lysis of red blood cells around a colony) and behavior on Chapman medium (Mannitol Salt Agar). Growth and fermentation on Chapman medium, where the phenol red indicator turns yellow, is a positive result indicating mannitol fermentation, a characteristic often associated with S. aureus.

Differentiation of Staphylococcus Species

The genus is broadly divided into two groups based on the production of the coagulase enzyme. Staphylococcus aureus is the primary coagulase-positive species. Other coagulase-positive species include S. intermedius and S. coagulans. The second group is known as CNS (Coagulase-Negative Staphylococci). While CNS species are part of the healthy human microbiota, they frequently cause opportunistic infections. Common CNS species include S. epidermidis, S. haemolyticus, S. hominis, S. warneri, S. capitis, S. lugdunensis, and S. saprophyticus.

Virulence Factors of Staphylococcus aureus

Staphylococcus aureus possesses an array of virulence factors that facilitate tissue colonization, immune evasion, and host damage. These are categorized into structural elements, enzymes, and toxins.

Structural elements and surface proteins include the capsule, slime layers (glycocalyx), teichoic acid, and lipoteichoic acid. A critical component is Protein A, which protects the bacterium by binding to the Fc fragment of immunoglobulins (IgG), effectively preventing opsonization. Surface proteins known as MSCRAMMs (Microbial Surface Components Recognizing Adhesive Matrix Molecules) are responsible for adhesion to host tissues by binding to fibronectin, fibrinogen, collagen, and elastin.

Enzymatic factors include coagulase and CF (clumping factor), which convert fibrinogen to fibrin; hyaluronidase, which breaks down connective tissue; fibrinolysin (staphylokinase), which dissolves blood clots; and various others like nuclease, protease, and lipase.

S. aureus also produces potent toxins: cytotoxins such as haemolysins and leukocidins (notably PVL - Panton-Valentine Leukocidin); exfoliatins (epidermolytic toxins); toxic shock syndrome toxin-1 (TSST-1); and various enterotoxins.

Local and Invasive Infections Caused by S. aureus

Staphylococcus aureus can cause infections in virtually all tissues and organs, ranging from superficial local infections to deep, invasive systemic diseases. Local purulent infections include furunculosis (boils), carbuncles (clusters of boils), folliculitis (specifically sycosis or styes in the eye), impetigo contagiosa (liszajec zakaźny), ecthyma (niesztowica), paronychia (zanokcica), felon (zastrzał), hidradenitis suppurativa (purulent inflammation of sweat glands), and various abscesses. It is also a frequent cause of surgical site infections, wound infections, and trauma-related complications.

Other specific conditions caused by S. aureus include mastitis (zapalenie sutka), conjunctivitis, otitis media, chronic sinusitis, peritonsillar abscesses, and pneumonia. Deep or invasive infections involve osteomyelitis (bone and marrow infection), endocarditis, and empyema of the pleura. S. aureus can lead to sepsis and secondary hematogenous infections in the lungs, musculoskeletal system, urogenital tract, and central nervous system. It can also cause intracranial venous thrombosis, often as a complication of sinusitis, mastoiditis, or subcutaneous scalp infections.

Toxin-Mediated Staphylococcal Diseases

Staphylococcal food poisoning is caused by enterotoxins, which are heat-stable proteins resistant to digestive enzymes. Approximately $35\%$ of S. aureus strains produce these toxins. Symptoms appear very rapidly, within $1$ to several hours after ingestion, and include vomiting, diarrhea, abdominal pain, sweating, and headache.

Staphylococcal Scalded Skin Syndrome (SSSS), also known as Ritter's disease, is caused by exfoliative toxins (ETA and ETB) produced by $5\%\text{ to }10\%$ of strains. These toxins target the superficial layers of the epidermis, causing the formation of large, sterile, serum-filled blisters that burst easily under mechanical stress (Nikolsky sign), exposing large areas of denuded skin. This leads to massive fluid loss and dehydration, with a high risk of secondary bacterial infection. This condition primarily affects neonates.

Toxic Shock Syndrome (TSS) is caused by TSST-1, a pyrogenic toxin whose gene is present in roughly $1\%$ of S. aureus strains. Toxin production occurs in microaerophilic conditions, such as those found in postoperative wounds or through the use of tampons during menstruation. Symptoms include high fever, hypotension, a scarlatiniform erythema, and sheet-like desquamation of the skin during recovery. Clinical diagnosis involves the involvement or damage of $\ge 3$ organs or systems, including peripheral blood/coagulation, the CNS, the gastrointestinal tract, the liver, mucous membranes, the muscular system, or the kidneys.

Epidemiology and Colonization

Staphylococcus aureus strains exist in the human population both constantly and periodically. They colonize the nostrils, nasopharynx, perianal region, armpits (pachy), groin (pachwiny), and the hairline on the forehead. The primary routes of transmission are direct contact (hands) and, more rarely, airborne via dust particles. Mandatory screening for S. aureus carriage is required for patients undergoing cardiac surgery, invasive cardiology procedures, and orthopedic procedures involving joint implants.

Coagulase-Negative Staphylococci (CNS) and Biofilm Formation

CNS are opportunistic pathogens and common members of the normal skin and mucosal microbiota. Staphylococcus epidermidis is the species most frequently isolated from clinical materials. These bacteria are primary etiologic agents in infections involving implanted foreign bodies, such as vascular catheters, joint prostheses, heart valves, cerebrospinal fluid shunts, and pacemakers. These are often categorized as BAI (Biomaterial-Associated Infection).

Staphylococcus saprophyticus is specifically noted for causing urinary tract infections and other opportunistic infections. The pathology of CNS infections is heavily linked to biofilm formation. This process begins with the contamination of a biomaterial during implantation or via transient bacteraemia. The stages of biofilm development include:

1. Initial Adhesion: Reversible attachment involving physical forces.
2. Irreversible Adhesion: Facilitated by adhesins and the production of slime (extracellular polymeric substances).
3. Maturation: Involving cell-to-cell signaling (quorum sensing), adhesin down-regulation, and matrix production.
4. Dispersal: Detachment of cells via signals or matrix degradation to colonize new sites.

Mechanisms of Antibiotic Resistance in Staphylococci

Staphylococci exhibit several critical resistance mechanisms, including the production of staphylococcal penicillinases, methicillin resistance, MLSB (macrolide-lincosamide-streptogramin B) resistance, and glycopeptide resistance. Strains that produce penicillinase but lack receptor-type resistance to beta-lactam antibiotics are termed MSSA (Methicillin-Susceptible Staphylococcus aureus) or MSCNS (Methicillin-Susceptible Coagulase-Negative Staphylococci).

Methicillin resistance (MRSA for S. aureus and MRCNS for coagulase-negative species) refers to receptor-based resistance to all beta-lactam antibiotics. This is historically named after methicillin, the first antibiotic against which this resistance was observed. MRSA is further classified into:

• HA-MRSA (Hospital-Acquired): Strains isolated from hospital infections.
• CA-MRSA (Community-Acquired): Strains isolated from community infections, often representing distinct clones, frequently carrying the Panton-Valentine Leukocidin (PVL) gene $\text{lukPV}$. These cause primary acute purulent skin infections (abscesses, boils) and occasionally necrotizing pneumonia following influenza.
• LA-MRSA/FA-MRSA (Livestock/Farm-Associated): Strains found in livestock (pigs, cattle, horses, poultry) and domestic animals (cats, dogs).

Epidemiological Classification and Trends

According to Klevens et al. (modified), healthcare-associated MRSA is divided into Hospital-onset (HO-MRSA), diagnosed after $48$ hours of hospitalization, and Community-onset (CO-MRSA), diagnosed within the first $48$ hours but with at least one documented risk factor (e.g., catheters, prior MRSA history, surgery, or dialysis within the last $12$ months). Community-associated (CA-MRSA) is diagnosed within the first $48$ hours with no healthcare risk factors.

In Poland, monitoring by the EARS-Net network shows that MRSA resistance levels in S. aureus from blood cultures have been systematically decreasing. In $2021$ (during the COVID-19 pandemic), the level was $16.5\%$. This dropped to $13\text{ to }14\%$ in $2022-2023$ and reached $10.8\%$ in $2024$. Similarly, the European weighted average has decreased from $18.2\%$ in $2019$ to $15.8\%$ in $2023$, and $14.2\%$ in $2024$.

Additional resistance data for S. aureus (sample size $n=2542$) in Poland for $2024$ includes:

• Gentamicin: $3.5\%$
• Ciprofloxacin: $29.0\%$
• Erythromycin: $26.6\%$
• Clindamycin: $11.9\%$
• Rifampicyn: $1.0\%$
• Linezolid: $0.0\%$
• Vancomycin: $0.0\%$