Young children with poor hygiene (impetigo and cutaneous infections).
Patients with intravascular catheters (bacteremia and endocarditis) or shunts (meningitis).
Patients with compromised pulmonary function or a viral respiratory infection (pneumonia).
MRSA is now the most common cause of community-acquired skin and soft-tissue infections.
Diseases
Toxin-mediated diseases:
Food poisoning.
Toxic shock syndrome.
Scalded skin syndrome.
Pyogenic diseases:
Impetigo.
Folliculitis.
Furuncles.
Carbuncles.
Wound infections.
Other systemic diseases.
Diagnosis
Microscopy is useful for pyogenic infections but not for blood or toxin-mediated infections.
Staphylococci grow rapidly on nonselective media.
Selective media (e.g., chromogenic agar, mannitol-salt agar) can recover S.aureus from contaminated specimens.
Nucleic acid amplification tests screen for MSSA and MRSA carriage.
Identification via biochemical tests (e.g., coagulase), molecular probes, or mass spectrometry.
Treatment, Prevention, and Control
Localized infections are managed by incision and drainage; systemic infections require antibiotic therapy.
Empirical therapy targets MRSA strains.
Oral antibiotics: trimethoprim-sulfamethoxazole, doxycycline/minocycline, clindamycin, or linezolid.
Intravenous therapy: vancomycin (drug of choice), daptomycin, tigecycline, or linezolid.
Treatment for food poisoning is symptomatic.
Proper wound cleansing and disinfectants prevent infections.
Thorough hand washing and covering exposed skin prevent spread.
Coagulase-Negative Staphylococci
Trigger Words
Opportunistic
Slime layer
Subacute
Biology and Virulence
Catalase-positive, coagulase-negative, gram-positive cocci arranged in clusters.
Relatively avirulent, but slime layer production allows adherence to foreign bodies (e.g., catheters, grafts, prosthetic valves/joints, shunts) and protection from phagocytosis and antibiotics.
Epidemiology
Normal human flora on skin and mucosal surfaces.
Can survive on dry surfaces for long periods.
Spread through direct contact or contaminated fomites, but most infections involve the patient’s own organisms.
Patients are at risk when a foreign body is present.
Ubiquitous, with no geographic or seasonal limitations.
Diseases
Infections include:
Subacute endocarditis.
Infections of foreign bodies.
Urinary tract infections.
Diagnosis
Similar to S.aureus infections.
Treatment, Prevention, and Control
Oxacillin (or other penicillinase-resistant penicillin) is used, or vancomycin for oxacillin-resistant strains.
Removal of the foreign body is often required.
Prompt treatment is necessary to prevent further tissue damage or immune complex formation.
General Characteristics of Staphylococci
Gram-positive cocci, spherical shape, no endospores.
Catalase-positive (differentiates from Streptococcus and Enterococcus).
Grow in grape-like clusters.
Size: 0.5 to 1.5 µm in diameter.
Can grow in aerobic and anaerobic conditions, high salt concentrations (e.g., 10% sodium chloride), and temperatures from 18°C to 40°C.
Over 80 species and subspecies found on human skin and mucous membranes.
Specific niches:
S.aureus: anterior nares.
S.capitis: sebaceous glands (e.g., forehead).
S.haemolyticus and S.hominis: apocrine glands (e.g., axilla).
Important human pathogens causing opportunistic infections and systemic diseases.
Most common species associated with human diseases: S.aureus, S.epidermidis, S.lugdunensis, and S.saprophyticus.
MRSA causes serious infections in hospitalized patients and healthy individuals.
S.aureus colonies can have a yellow or gold color due to carotenoid pigments.
S.aureus produces coagulase, which converts fibrinogen to fibrin, forming a clot when suspended in plasma.
Coagulase-negative staphylococci are less virulent and cause opportunistic infections.
Physiology and Structure of Staphylococci
Capsule and Slime Layer
Polysaccharide capsule covers the cell wall.
Serotypes 1 and 2: thick capsules, mucoid colonies, rarely associated with human disease.
Serotypes 5 and 8: associated with ~75% of human infections.
Capsule protects bacteria by inhibiting phagocytosis by polymorphonuclear leukocytes (PMNs).
Slime layer (biofilm): water-soluble film of monosaccharides, proteins, and small peptides produced by staphylococci.
Extracellular substance binds bacteria to tissues and foreign bodies, important for coagulase-negative staphylococci survival.
Peptidoglycan and Associated Enzymes
Peptidoglycan forms half the cell wall by weight.
Layers of glycan chains with N-acetylmuramic acid and N-acetylglucosamine subunits.
Oligopeptide side chains are attached to N-acetylmuramic acid subunits and cross-linked with peptide bridges.
Gram-positive bacteria have many cross-linked layers for rigidity.
Penicillin-binding proteins (PBPs) catalyze peptidoglycan layer construction and are targeted by penicillins and β-lactam antibiotics.
Resistance to methicillin and related antibiotics is mediated by the mecA and mecC genes encoding PBP2a, which has low affinity for these antibiotics.
mecA gene is on the staphylococcal cassette chromosome mec (SCCmec).
MRSA strains are now present in the community, not just hospitals.
Peptidoglycan has endotoxin-like activity, stimulating endogenous pyrogens, complement activation, IL-1 production, and PMN aggregation.
Teichoic Acids and Lipoteichoic Acids
Teichoic acids are species-specific, phosphate-containing polymers bound either covalently to N-acetylmuramic acid residues of the peptidoglycan layer or to lipids in the cytoplasmic membrane (lipoteichoic acids).
Stimulate specific antibody response when bound to peptidoglycan.
Surface Adhesion Proteins
Important virulence factors that adhere to host matrix proteins.
Staphylococcal protein A (spa): binds to the Fc receptor of IgG1, IgG2, and IgG4.
Fibronectin-binding protein A: binds fibronectin.
S. aureus surface protein A: undetermined function.
MSCRAMM proteins include staphylococcal protein A, fibronectin-binding proteins A and B, and clumping factor proteins A and B.
Clumping factor proteins (coagulase) bind fibrinogen and convert it to insoluble fibrin, causing staphylococci to clump/aggregate.
Cytoplasmic Membrane
Made up of a complex of proteins, lipids, and a small amount of carbohydrates.
Serves as an osmotic barrier for the cell and provides an anchorage for the cellular biosynthetic and respiratory enzymes.
Pathogenesis and Immunity
Ability to evade immune clearance.
Produce surface proteins that mediate adherence to host tissues during colonization.
Produce disease through specific toxins and hydrolytic enzymes, leading to tissue destruction.
These properties (immunologic evasion, adherence, tissue destruction) are common to most pathogenic organisms.
Regulation of Virulence Genes
Expression of virulence factors and biofilm formation is controlled by the accessory gene regulator (agr) operon.
Quorum-sensing system allows expression of adhesion proteins and promotes tissue colonization and intracellular growth when bacterial density is low, and tissue invasion/toxin production when density is high.
Operon encodes autoinducer peptides (AIP1 to 4) that bind to cell-surface receptors and regulate protein expression based on population density.
Innate immune regulation of bacterial virulence is mediated by apolipoprotein B, which binds to AIPs and suppresses agr signaling.
Bacterial density is maintained at a low concentration, providing the benefits of immune stimulation by colonizing staphylococci without tissue invasion/destruction.
Defenses against Innate Immunity
Opsonins bind to encapsulated staphylococci, but the capsule inhibits phagocytosis by PMNs.
Specific antibodies increase C3 binding, leading to phagocytosis.
Extracellular slime layer interferes with phagocytosis.
Protein A binds immunoglobulins, preventing antibody-mediated immune clearance and forming immune complexes.
Staphylococcal Toxins
S.aureus produces:
5 cytolytic or membrane-damaging toxins (alpha, beta, delta, gamma, and P-V leukocidin).
2 exfoliative toxins (A and B).
Numerous enterotoxins (A to E, G to X, plus multiple variants).
TSST-1.
Cytolytic toxins = hemolysins, but activity is not restricted to red blood cells, and P-V leukocidin cannot lyse erythrocytes.
Cytotoxins can lyse neutrophils, resulting in the release of lysosomal enzymes that subsequently damage surrounding tissues.
P-V leukocidin is linked to severe pulmonary and cutaneous infections.
Exfoliative toxin A, enterotoxins, and TSST-1 are superantigens that bind to class II MHC molecules on macrophages, interacting with T-cell receptors (VβTCR).
This results in massive cytokine release by macrophages (IL-1β and TNF-α) and T cells (IL-2, IFN-γ, and TNF-β).
IL-1β is associated with fever.
TNF-α and TNF-β are associated with hypotension and shock.
Cytotoxins
Alpha toxin, encoded on the bacterial chromosome and a plasmid, is a 33,000-Da polypeptide produced by most strains causing human disease.
Disrupts smooth muscle in blood vessels and is toxic to erythrocytes, leukocytes, hepatocytes, and platelets.
Binds to the cell surface, aggregates into a heptamer with a 1- to 2-nm pore, causing rapid efflux of K+ and influx of Na+, Ca2+, and other small molecules, leading to osmotic swelling and cell lysis.
Important mediator of tissue damage in staphylococcal disease.
Beta toxin (sphingomyelinase C) is a 35,000-Da heat-labile protein produced by most strains.
Specificity for sphingomyelin and lyso-phosphatidylcholine, toxic to erythrocytes, fibroblasts, leukocytes, and macrophages.
Catalyzes hydrolysis of membrane phospholipids, with lysis proportional to sphingomyelin concentration.
Effect on erythrocytes occurs primarily at low temperatures.
Delta toxin is a 3000-Da polypeptide produced by almost all S.aureus strains and other staphylococci.
Wide spectrum of cytolytic activity, disrupting cellular membranes through detergent-like action.
Gamma toxin and P-V leukocidin:
Bicomponent toxins composed of two polypeptide chains: S (slow-eluting proteins) and F (fast-eluting proteins) components.
Three unique S proteins (HlgA, HlgC, and LukS-PV) and two F proteins (HlgB and LukF-PV) have been identified.
All six toxins can lyse neutrophils and macrophages.
Greatest hemolytic activity is associated with HlgA/HlgB, HlgC/HlgB, and HlgA/LukF-PV.
PV leukocidin toxin (LukS-PV/LukF-PV) is leukotoxic but has no hemolytic activity.
Cell lysis is mediated by pore formation, with increased permeability to cations and osmotic instability.
Exfoliative Toxins
SSSS is mediated by exfoliative toxins, prevalent in less than 5% of S.aureus strains.
Two distinct forms: ETA (heat stable, phage-associated) and ETB (heat labile, plasmid-located).
Serine proteases split desmoglein-1, responsible for intercellular bridges in the stratum granulosum epidermis.
No cytolysis or inflammation, neither staphylococci nor leukocytes are present in the involved layer of the epidermis (diagnostic clue).
Protective neutralizing antibodies develop after exposure.
Seen mostly in young children.
Enterotoxins
Enterotoxin A is most commonly associated with food poisoning.
Enterotoxins C and D are found in contaminated milk products.
Enterotoxin B causes staphylococcal pseudomembranous enterocolitis.
Stable to heating at 100°C for 30 minutes and resistant to hydrolysis by gastric and jejunal enzymes.
Produced by 30% to 50% of all S.aureus strains.
Superantigens capable of inducing nonspecific activation of T cells and massive cytokine release.
Histologic changes include neutrophil infiltration and loss of the brush border in the jejunum.
Stimulation of inflammatory mediator release from mast cells causes emesis.
Toxic Shock Syndrome Toxin-1
TSST-1 is a 22,000-Da heat- and proteolysis-resistant, chromosomally mediated exotoxin.
Produced by 90% of S.aureus strains responsible for menstruation-associated TSS and half of the strains responsible for other forms of TSS.
Enterotoxin B and enterotoxin C are responsible for approximately half the nonmenstruation-associated TSS cases.
Requires elevated oxygen concentration and neutral pH.
Superantigen stimulating cytokine release, producing leakage of endothelial cells and cytotoxic effects.
Coagulase-negative staphylococci are present on the skin, and transient colonization of moist skinfolds with S.aureus is common.
Colonization of the umbilical stump, skin, and perineal area of neonates with S.aureus is common.
S.aureus and coagulase-negative staphylococci are found in the nares, oropharynx, gastrointestinal tract, and urogenital tract.
Approximately 15% of normal healthy adults are persistent nasopharyngeal carriers of S.aureus, higher incidence for hospitalized patients, medical personnel, and those with eczematous skin diseases.
Adherence of the organism to the mucosal epithelium is regulated by the staphylococcal cell-surface adhesins.
Shedding of bacteria is common and is responsible for many hospital-acquired infections.
Staphylococci are susceptible to high temperatures and disinfectants but can survive on dry surfaces.
Transferred to susceptible persons through direct contact or fomites.
MRSA spread rapidly in hospitalized patients starting in the 1980s.
Community-acquired MRSA outbreaks were reported in 2003, with genetically unique strains.
Clinical Diseases
Staphylococcus Aureus
Clinical manifestations result from toxin activity (SSSS, staphylococcal food poisoning, and TSS) or proliferation of organisms (cutaneous infections, endocarditis, pneumonia, empyema, osteomyelitis, septic arthritis).
Introduction of small numbers of staphylococci can establish disease in the presence of a foreign body.
Patients with impaired chemotactic or phagocytic response are more susceptible to staphylococcal diseases.
Staphylococcal Scalded Skin Syndrome
Ritter disease or SSSS is characterized by the abrupt onset of perioral erythema that spreads over the entire body within 2 days.
Slight pressure displaces the skin (positive Nikolsky sign), and large bullae form, followed by desquamation of the epithelium.
Blisters contain clear fluid but no organisms or leukocytes.
The epithelium becomes intact again within 7 to 10 days, when antibodies against the toxin appear.
Scarring does not occur because only the top layer of epidermis is affected.
Primarily a disease of neonates and young children, with a mortality rate less than 5%.
Infections in adults usually occur in immunocompromised hosts or patients with renal disease.
Bullous impetigo is a localized form of SSSS, with specific strains of toxin-producing S.aureus, associated with formation of superficial skin blisters.
S.aureus is present in the localized blisters of patients with bullous impetigo.
Erythema does not extend beyond the borders of the blister, and the Nikolsky sign is not present.
Occurs primarily in infants and young children and is highly communicable.
Staphylococcal Food Poisoning
Intoxication rather than an infection, caused by bacterial toxin present in food.
Results from contamination of food by a human carrier.
After staphylococci are introduced into the food, the food must remain at room temperature or warmer for the organisms to grow and release the toxin.
The contaminated food will not appear or taste tainted, and subsequent heating will kill the bacteria but not inactivate the heat-stable toxin.
Abrupt and rapid onset, with a mean incubation period of 4 hours.
Severe vomiting, diarrhea, and abdominal pain or nausea are characteristic.
Sweating and headache may occur, but fever is not seen.
Diarrhea is watery and non-bloody.
Treatment is for relief of abdominal cramping and diarrhea and for fluid replacement; antibiotic therapy is not indicated.
Short-lived immunity means that second episodes can occur, particularly with serologically distinct enterotoxins.
Certain strains can also cause enterocolitis, manifested by watery diarrhea, abdominal cramps, and fever.
Enterocolitis occurs primarily in patients who have received broad-spectrum antibiotics.
Diagnosis is confirmed after more common causes have been excluded, and abundant S.aureus is detected in the stool.
Toxic Shock Syndrome
Initiated with the localized growth of toxin-producing strains in the vagina or a wound, followed by release of the toxin into the blood.
Toxin production requires an aerobic atmosphere and neutral pH.
Clinical manifestations start abruptly and include fever, hypotension, and a diffuse, macular, erythematous rash.
Multiple organ systems are involved, and the entire skin desquamates.
Purpura fulminans is a particularly virulent form of TSS, characterized by large purpuric skin lesions, fever, hypotension, and disseminated intravascular coagulation.
Risk of recurrent disease is as high as 65% unless the patient is specifically treated with an effective antibiotic.
More than 50% of patients fail to develop protective antibodies after their disease resolves.
Cutaneous Infections
Localized pyogenic cutaneous infections include impetigo, folliculitis, furuncles, and carbuncles.
Impetigo is a superficial infection that mostly affects young children, occurring primarily on the face and limbs.
Folliculitis is a pyogenic infection in the hair follicles.
Furuncles (boils) are an extension of folliculitis.
Carbuncles occur when furuncles coalesce and extend into the deeper subcutaneous tissue, accompanied by chills and fevers.
Staphylococcal wound infections occur after a surgical procedure or trauma when organisms colonizing the skin or from an external source are introduced into the wound.
Infections are characterized by edema, erythema, pain, and an accumulation of purulent material.
MRSA strains are now the most common cause of skin and soft-tissue infections in patients presenting to U.S. hospital emergency departments.
Bacteremia and Endocarditis
S.aureus is a common cause of bacteremia.
The initial foci of infection are not known in approximately a third of patients.
Acute endocarditis caused by S.aureus is a serious disease, with a mortality rate approaching 50%.
Symptoms can deteriorate rapidly and include disruption of cardiac output and peripheral evidence of septic embolization.
The exception is S.aureus endocarditis in parenteral drug abusers, whose disease normally involves the right side of the heart.
Pneumonia and Empyema
S.aureus respiratory disease can develop after the aspiration of oral secretions or from the hematogenous spread of the organism from a distant site.
Radiographic examination reveals the presence of patchy infiltrates with consolidation or abscesses.
Community-acquired MRSA is responsible for a severe form of necrotizing pneumonia with massive hemoptysis, septic shock, and a high mortality rate.
Empyema occurs in 10% of patients with pneumonia, and S.aureus is responsible for a third of all cases.
Osteomyelitis and Septic Arthritis
Osteomyelitis results from hematogenous dissemination to bone or a secondary infection resulting from trauma or the extension of disease from an adjacent area.
Hematogenous spread in children generally results from a cutaneous staphylococcal infection and usually involves the metaphyseal area of long bones.
In adults, it commonly occurs in the form of vertebral osteomyelitis.
With appropriate antibiotic therapy and surgery, the cure rate for staphylococcal osteomyelitis is excellent.
S.aureus is the primary cause of septic arthritis in young children and in adults.
Staphylococcal arthritis is characterized by a painful erythematous joint, with purulent material obtained on aspiration.
Staphylococcus Epidermidis and Other Coagulase-Negative Staphylococci
S.epidermidis, S.lugdunensis, can infect prosthetic and less commonly, native heart valves.
Infections of native valves are believed to result from the inoculation of organisms onto a damaged heart valve.
Catheter and shunt infections: infections of catheters and shunts are caused by coagulase-negative staphylococci.
Prosthetic joint infections: infections of artificial joints caused by coagulase-negative staphylococci.
Urinary Tract Infections: S.saprophyticus causes urinary tract infections in young, sexually active women.
Laboratory Diagnosis
Microscopy
Staphylococci are gram-positive cocci.
Successful detection depends on the type of infection and the quality of the material submitted for analysis.
Nucleic Acid–Based Tests
Used to detect nasal carriage of methicillin-sensitive S.aureus (MSSA) and MRSA.
Culture
Clinical specimens should be inoculated onto nutritionally enriched agar media supplemented with sheep blood.
Staphylococci grow rapidly on nonselective media incubated aerobically or anaerobically.
Identification
Biochemical tests used to identify S.aureus. Colonies resembling S.aureus are identified in most laboratories by mixing a suspension of organisms with a drop of plasma. Mass spectrometry has been used to identify the staphylococci. Analysis of genomic DNA by pulsed-field gel electrophoresis a method for characterizing isolates. Whole genome sequencing is becoming. the preferred tool for subtyping organisms.
Antibody Detection
Antibodies to cell wall teichoic acids are present in many patients with long-standing S.aureus infections.
Treatment, Prevention, and Control
Less than 10% of the strains are susceptible to penicillin.
Currently, the majority of S.aureus responsible for hospital- and community-acquired infections are resistant to semisynthetic penicillins, and these MRSA strains are resistant to all β-lactam antibiotics.
Patients with localized skin and soft-tissue infections can generally be managed by incision and drainage of the abscesses.
Oral therapy can include trimethoprim-sulfamethoxazole, a long-acting tetracycline such as doxycycline or minocycline, clindamycin, or linezolid.
Vancomycin is the drug of choice for intravenous therapy, with daptomycin, tigecycline, or linezolid acceptable alternatives.
Isolates of S.aureus have been found with low-level resistance which is observed in strains with a thicker, more disorganized cell wall and high-level resistance to vancomycin. Staphylococci are ubiquitous organisms present on the skin and mucous membranes.
The number of organisms required to establish an infection is generally large unless a foreign body is present in the wound.
The spread of staphylococci from person to person is more difficult to prevent.