Staphylococci PDF - Microbiology Lecture Notes 2024-2025

Staphylococci Dr. Mohammed Ameer Abdullah Ninevah University College of Medicine Department of Medical Microbiology 2024-2025 Learning Objectives Species of Staphylococcus. Morphology and Culture characteristics of Staphylococcus aureus. Virulence factors of Stap...

Staphylococci Dr. Mohammed Ameer Abdullah Ninevah University College of Medicine Department of Medical Microbiology 2024-2025 Learning Objectives Species of Staphylococcus. Morphology and Culture characteristics of Staphylococcus aureus. Virulence factors of Staphylococcus aureus. Laboratory diagnosis of infections caused by Staphylococcus aureus. Coagulase-negative staphylococci (CNS). Treatments. Staphylococci The staphylococci are Gram-positive spherical cells, usually arranged in grapelike clusters. They grow readily on many types of media, fermenting carbohydrates and producing pigments that vary from white to deep yellow. Some are members of the normal microbiota of the skin and mucous membranes of humans; others cause abscess formation, a variety of pyogenic infections, and even fatal septicemia. The genus Staphylococcus has at least 45 species. The most frequently species of clinical importance are Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus saprophyticus. Staphylococcus aureus, is one of the most common causes of bacterial infections, and is also an important cause of food poisoning and toxic shock syndrome. Among less virulent staphylococcal species, Staphylococcus epidermidis is an important cause of prosthetic implant infections, whereas Staphylococcus saprophyticus causes urinary tract infections, especially cystitis in women. GENERAL FEATURES Staphylococci generally stain gram positive. They are round rather than oval and tend to occur in bunches like grapes. Staphylococci are facultatively anaerobic organisms. They produce catalase, which is one feature that distinguishes them from the catalase- negative streptococci. The most virulent species of staphylococcus is S. aureus, almost all isolates of which secrete coagulase, an enzyme that causes plasma to clot. Other species that cause disease and lack coagulase are often referred to as Coagulase Negative Staphylococci. Staphylococci are hardy, being resistant to heat and drying, and thus can persist for long periods on fomites (inanimate objects), which can then serve as sources of infection. Staphylococcus aureus Generally, significant host compromise is required for S. aureus infection, such as a break in the skin or insertion of a foreign body (for example, wounds, surgical infections, or central venous catheters), an obstructed hair follicle (folliculitis), or a compromised immune system. S. aureus disease may be: 1) The result of actual invasive infection, overcoming host defense mechanisms, and the production of extracellular substances which facilitate invasion; 2) a result of toxins in the absence of invasive infection ; or 3) a combination of invasive infection and intoxication. Epidemiology S. aureus is carried by healthy individuals on the skin and mucous membranes. Carriers serve as a source of infection to themselves and others; for example, by direct contact, contamination of fomites or contamination of food, which can then result in food poisoning. Pathogenesis Virulence factors are the genetic, biochemical, or structural features that enable an organism to produce disease. The clinical outcome of an infection depends on the virulence of the pathogen and the opposing effectiveness of the host defense mechanisms. S. aureus expresses many potential virulence factors. For the majority of diseases caused by S.aureus, pathogenesis depends on the combined actions of several virulence factors, so it is difficult to determine the role of any given factor. 1. Cell wall virulence factors: a. Capsule: Most clinical isolates express a polysaccharide “microcapsule”. The capsule layer is very thin but has been associated with increased resistance to phagocytosis. b. Peptidoglycan: Provides osmotic stability; stimulates production of endogenous pyrogen (endotoxin-like activity); leukocyte chemoattractant (abscess formation); inhibits phagocytosis. c. Teichoic acid : Binds to fibronectin. d. Protein A: is a major component of the S. aureus cell wall. It binds to the Fc region of IgG, exerting an anti-opsonin (and therefore strongly antiphagocytic effect). e. Fibronectin-binding protein: Fibrinectin-binding protein (FnBP) and other staphylococcal surface proteins promote binding to mucosal cells and tissue matrixe. f. Clumping factor: is a fibrinogen-binding protein present on the surface of S.aureus that binds to fibrinogen converts it to insoluble fibrin causing the staphylococci to clump. 2. Enzymes S. aureus produces an extracellular coagulase, an enzyme like protein that clots plasma. Coagulase binds to prothrombin; together they become enzymatically active and initiate fibrin polymerization. Coagulase may deposit fibrin on the surface of staphylococci, perhaps altering their ingestion by phagocytic cells or their destruction within such cells. Catalase this enzyme converts H2O2 to water, and oxygen. Hyaluronidase (spreading factor) it hydrolyzes hyluronic acid (a component of extracellular matrix of connective tissues) though promoting the spread of bacteria. Fibrinolysin, also called staphylokinase, can dissolve fibrin clots. Lipases that hydrolyze lipids and ensure survival of staphylococci in the sebaceous areas of the body. Nuclease (DNAase) that can hydrolyze viscous deoxyribonucleic acid (DNA). 3. Toxins Cytolytic exotoxins: α, β, γ, and δ Toxins attack mammalian cell (including RBC) membranes, and are often referred to as hemolysins. Panton-Valentine leukocidin: This pore-forming toxin lyses Polymorphonuclear leukocytes (PMNs). Production of this toxin makes strains more virulent. Superantigen exotoxins: These toxins have an affinity for the T-cell receptor– major histocompatibility complex Class II antigen complex. They stimulate enhanced T-lymphocyte response. This major T-cell activation can cause toxic shock syndrome, primarily by release into the circulation large amounts of T-cell cytokines, such as interleukin-2 (IL- 2), interferon-γ (IFN-γ), and tumor necrosis factor-α (TNF-α). A. Enterotoxins: Enterotoxins are produced by approximately half of all S.aureus isolates. When these bacteria contaminate food and are allowed to grow, they secrete enterotoxin, ingestion of which can cause food poisoning. B. Toxic shock syndrome toxin (TSST –1): This is the classic cause of toxic shock syndrome (TSS). the toxin are superantigens. C. Exfoliatin (ET) is also a superantigen. It causes scalded skin syndrome in children. The toxin attacks the intercellular adhesive of the stratum granulosum, causing marked epithelial desquamation. Cleavage results in loss of the superficial skin layer. Virulence factors that may play a role in the pathogenesis of staphylococcal infections. Clinical significance S. aureus causes disease by infecting tissues. A common entry point into the body is a break in the skin, which may be a minute needlestick or a surgical wound. Another portal of entry is the respiratory tract. For example, staphylococcal pneumonia is a important complication of influenza. The localized host response to staphylococcal infection is inflammation, characterized by swelling, accumulation of pus, and necrosis of tissue. Fibroblasts and their products may form a wall around the inflamed area, which contains bacteria and leukocytes. This creates a characteristic pus- filled abscess. Serious consequences of staphylococcal infections occur when the bacteria invade the bloodstream. The resulting septicemia (the presence and persistence of pathogenic microorganisms or their toxins in the blood) may be rapidly fatal. Bacteremia (the presence of viable bacteria circulating in the bloodstream) may result in seeding internal abscesses, skin lesions, or infections in the lung, kidney, heart, skeletal muscle, or meninges. 1. Localized skin infections: The most common S. aureus infections are small, superficial abscesses involving hair follicles (folliculitis) or sweat or sebaceous glands Subcutaneous abscesses called furuncles (boils) often form around foreign bodies such as splinters.. Carbuncles are larger, deeper, multiloculated skin infections that can lead to bacteremia and require antibiotic therapy and debridement. Impetigo is usually a localized, superficial, spreading crusty skin lesion generally seen in children. folliculitis furuncles Carbuncles Impetigo 2. Deep, localized infections: These may be metastatic from superficial infections or may result from trauma. S. aureus is the most common cause of acute and chronic infection of bone marrow. S. aureus is also the most common cause of acute infection of joint space in children (Septic arthritis). 3. Acute endocarditis: is caused by injection of contaminated preparations or by needles contaminated with S. aureus. 4. Septicemia is a generalized infection with sepsis. 5. Pneumonia: S. aureus is a cause of severe, necrotizing pneumonia. 6. Nosocomial infections: S. aureus is one of the most common causes of hospital-associated infections, often of wounds or bacteremia associated with catheters. Progressive to septicemia is often a terminal event. 7. Toxinoses: These are diseases caused by the action of a toxin. Toxinoses caused by S. aureus include: a. Toxic shock syndrome: TSS results in high fever, rash , vomiting, diarrhea, hypotension, and multiorgan involvement damage (especially Gastrointestinal , renal and hepatic). b. Staphylococcal gastroenteritis: This is caused by ingestion of food contaminated with enterotoxin-producing S.aureus. Symptoms, such as nausea, vomiting, and diarrhea, are acute following a short incubation period (less than 6 hours) and are triggered by local actions of the toxin on the GI tract rather than from infection. c. Scalded skin syndrome: This involves the appearance of superficial bullae resulting from the action of an exfoliative toxin that attacks the intercellular adhesive of the stratum granulosum, causing marked epithelial desquamation. Laboratory identification Identification of an isolate as a staphylococcus relies largely on microscopic and colony morphology and catalase positivity. Bacteria stain strongly gram-positive, and are frequently seen in grapelike clusters. S. aureus is distinguished from the coagulase-negative staphylococci primarily by coagulase positivity. S. aureus colonies tend to be yellow and hemolytic , rather than gray and nonhemolytic like the coagulase-negative staphylococci. S. aureus is also distinguished from most coagulase-negative staphylococci by being mannitol-positive. Automated systems such as VITEK can also be used for rapid identification.  Catalase Test (Positive ) Coagulase Test (Positive) Negative Positive Positive Negative Slide method Tube method Treatment Serious S. aureus infections require aggressive treatment, including incision and drainage of localized lesions, as well as systemic antibiotics. Choice of antibiotics is complicated by the frequent presence of acquired antibiotic resistance determinants. Virtually all community and hospital-acquired S. aureus infections are now resistant to penicillin G due to penicillinase-encoding plasmids or transposons. This has required the replacement of the initial agent of choice, penicillin G, by β-lactamase-resistant penicillins, such as methicillin or oxacillin. However, increased use of methicillin and related antibiotics has resulted in S. aureus that is resistant to a number of β-lactam antibiotics, such as methicillin, oxacillin and amoxicillin. These strains are known as methicillin-resistant S.aureus. 1. Hospital-acquired methicillin-resistant S. aureus (MRSA): In recent decades, a high percentage (often in the range of 50 percent) of hospital S.aureus isolates has been found to be also resistant to methicillin or oxacillin. Antibiotic resistance is caused by chromosomal acquisition of the gene for a distinct penicillin binding protein (PBP), PBP-2a. This protein codes for a new Peptidoglycan Transpeptidase with a low affinity for all currently available β- lactam antibiotics, and thus renders infections with MRSA unresponsive to β- lactam therapy. MRSA infections are associated with worse outcomes, including longer hospital and intensive care unit stays, and higher mortality rates. MRSA strains are also frequently resistant to many other antibiotics, some being sensitive only to glycopeptides such as vancomycin. 2. Community-acquired MRSA (CA-MRSA): Community acquired MRSA infections were documented in the mid-1990s, occurring in individuals who had no previous risk factors for MRSA infections, such as exposure to hospital. The most common clinical manifestations of CA-MRSA are skin and soft tissue infections such as abscesses or cellulitis. Less commonly, CA-MRSA can also cause severe diseases such as necrotizing pneumonia, osteomyelitis, and septicemia. 3. Vancomycin resistance: Vancomycin has been the agent of choice for treatment of life-threatening MRSA S. aureus infections. in 1997, several MRSAs were isolated that had also acquired low-level vancomycin resistance. The incidence of vancomycin resistance has increased steadily, prompting the use of alternative drugs such as linezolid and daptomycin. These agents have good in vitro activity against MRSA and most other clinically important gram-positive bacterial pathogens. Prevention There is no effective vaccine against S. aureus. Infection control procedures, such as disinfection of hands and fomites, are important in the control of nosocomial S. aureus epidemics. COAGULASE-NEGATIVE STAPHYLOCOCCI (CNS) CNS species are important agents of hospital-acquired infections associated with the use of implanted prosthetic devices and catheters. Staphylococcus epidermidis S. epidermidis is present in large numbers as part of the normal flora of the skin. Despite its low virulence, it is a common cause of infection of implants such as heart valves and catheters. vancomycin-resistant isolates have been reported. S. epidermidis produces an extracellular polysaccharide material called polysaccharide intercellular adhesin , that facilitates adherence to bioprosthetic material surfaces, such as intravenous catheters, and acts as a barrier to antimicrobial agents. Staphylococcus saprophyticus This organism is a frequent cause of cystitis in women, probably related to its occurrence as part of normal vaginal flora. It tends to be sensitive to most antibiotics, even penicillin G. S. saprophyticus can be distinguished from S. epidermidis and most other coagulase- negative staphylococci by its natural resistance to novobiocin. it is concluded that the Van@citrate-AgNP is more potent than the single entity vancomycin towards both S. aureus and E.coli Thank you