Bacteriology-32 Staphylococcal infections*

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What are the general characteristics of Staphyloccus species: Gram postive cocci in clusters
Aerobic/facultatively anaerobic
Nonmotile
Non-spore forming
Catalase positive
What are the three medically relevant Staphylococcus species: S. Aureus, S. epidermidis, S. saprophyticus.
S. Aureus is the most virulent
How are Staphylococci distinguished from Streptococci: Staphylococci are catalase positive
How is Staphylococcus aureus distinguished from other Staphylococcus species: S. aureus is coagulase positive. Coagulase leads to the development of a fibrin clot upon incubation of the bacterium with mammalian plasma
Why do Staphylococci present a serious problem for hospitals: They quickly develop antibiotic resistance
What is the natural habitat/transmission of Staphylococci: Skin and mucous membranes, within the anterior nares, perineum or urinary tract
How do most Staphylococci infections occur: Autoinfection. Community epidemics are from poor hygiene or fomite transmission. Hospital epidemics are from passage between patients or workers
How well do Staphylococci do in the environment: They are relatively resistant to adverse environments. They can survive for weeks on dried pus or sputum
What is responsible for whether or not one is a carrier of Staphylococci: Host factors, rather than strain-specific factors. Carriers can be persistent or intermittent
Give a brief overview of Staphylococcus aureus (catalase positive staphylococci): Mild to life threatening infections. Suppuration and necrosis of local tissue and a tendency for the infected area to become walled off with the formation of a pus-filled local abscess. Toxin-mediated disease, from food poisoning to toxic shock syndrome
What are some skin infections caused by Staphylococcus aureus: Stye: eyelash infection
Furnuncle (boil): in hair follicle, sweat gland, or sebaceous gland
Chronic furnunculosis: repeated boils; acne predisposes
Carbuncles (subcutaneous abcess): Spread from furnuncle
Impetigo: large blisters in the superficial layers of the skin
Describe Impetigo: Staphylococcus aureus is isolated along with Group A Strep in about 30% of pustular impetigo; Exfoliatin-producing strains cause bullous impetigo (large skin blisters) with is highly contagious
What are some common diseases caused by Staphylococcus aureus: Skin infections; Wound infections; Invasive infections (very serious) such as Deep lesions and Pneumonia; Toxin mediated diseases
Describe wound infections caused by Staphylococcus aureus: Can be very severe. Either acquired via auto-inoculation or via transmission from medical personnel. The hallmark of Staphylococcus infection is the presence of pus. 80% of suppurative infections seen in medicine are due to Staphylococcus aureus
What are the characteristics of deep lesions caused by Staphylococcus aureus: Can lead to osteomyelitis and septic arthritis; major cause of osteomyelitis in children
Endocarditis, septicemia, and meningitis can occur
Main risk group is those with decreased host defenses. Common among IV drug abusers
What are the characteristics of Pneumonia caused by Staphylococcus aureus: Usually secondary to trauma such as lung aspiration and intubation; also secondary to influenza. Cystic fibrosis patients are at risk
What are the toxin-mediated diseases caused by Staphylococcus aureus: Food poisoning; Impetigo; Scalded skin syndrome; Toxic Shock Syndrome
What are the characteristics of food poisoning caused by Staphylococcus aureus: Ingestion of pre-formed heat-stable enterotoxin
Explosive onset with acute vomiting and diarrhea 1-5 hours; no fever
High attack rate; potato salad and other creamy dishes; must refrigerate to prevent
What is bullous impetigo caused by: Staphylococcus strains that produce exfoliatin A
What is characteristic of nonbullous impetigo caused by Staphylococcus aureus: More severe in terms of number and severity of lesions if infecting strain produces exfoliatin B (also Panton-Valentine Leukocidin)
What are the characteristics of Scalded skin syndrome caused by Staphylococcus aureus: Exfoliatin is spread hematogenously to affect distant sites
Erythema, bullous formation, desquamation of entire sheets of epithelial cells starting with the face
Most common in children <5
What are the characteristics of Toxic Shock Syndrome caused by Staphylococcus aureus: Most cases are caused by TSST-1. Can act as a superantigen and cause massive release of cytokines
Menstrual TSS accounts for 55% of cases
High fever, vomiting, diarrhea, sore throat, muscle pain. Shock within 48 hrs. Skin rash at a deeper layer than scalded skin syndrome
What are the five surface components that enable Staphylococcus to cause invasive disease: Peptidoglycan: Septic shock if in bloodstream. Interspersed with ribitol-techoic acid, with is antigenic and binds to fibronectin
Protein A: Antiphagocytic by making Fc of IgG unavailable to phagocytes
Capsule: Impairs phagocytosis
Clumping factor: Receptor for binding fibrinogen
Cell surface bound coagulase: Helps coat cell with fibrin, increasing resistance to phagocytosis
What are the seven extracellular bacterial products that enable Staphylococcus aureus to cause invasive disease: Catalase; Extracellular coagulase; Cytotoxins; Exfoliatin A and B; Hyaluronidase; Enterotoxin; TSST-1
What does catalase, produced by Staphylococcus aureus, do: Neutralizes H2O2 produced by phagocytes
What does extracellular coagulase, produced by Staphylococcus aureus, do: Converts fibrinogen to fibrin to cause clot formation in plasma; walls of bacteria in lesion
What does Alpha hemolysin (alpha toxin), produced by Staphylococcus aureus, do: Lysis of RBCs, WBCs, platelets via formation of transmembrane pores. Similiar to complement, Streptolysin O, and cytotoxic T cell effector proteins.
What are the main cytotoxins produced by Staphylococcus aureus: Alpha hemolysin (alpha toxin); Beta, gamma, and delta cytotoxins; Panton-Valentine leukocidin (PVL)
How does Panton-Valentine leukocidin (PVL), produced by Staphylococcus aureus, work: Permeablizes and kills PMNs and macrophages by creating a pore in the membrane. Poor prognosis
What are Exfoliatin A and B responsible for: Bullous (type A) and more serious forms of nonbullous impetigo (type B). Also responsible for scalded skin syndrome. Intercellular splitting of the epidermis via interrupting desmosomal linkages of the stratum granulosum
What does Hyaluronidase, produced by Staphylococcus aureus, do: Hydrolyzes hyaluronic acid (acellular matrix of connective tissue) which facilitates movement of Staphylococcus aureus through tissue
What are the characteristics of the Staphylococcus aureus enterotoxin: Agent of staphylococcal food poisoning. Causes vomiting and diarrhea. Five serological types which resist boiling
What is the epidemiology of Staphylococcus aureus: Isolated infections are generally autoinfections. Impetigo outbreaks are usually to due to poor hygiene among children
Hospital outbreaks are from workers or patients
Food poisoning is from infected food handlers with a skin lesion.
Staph infections are a major cost for hospitals
People working in wet conditions are at high risk
What is the use of bacteriophage typing in Staphylococcus aureus infections: Most strains of S. aureus are lysogenic and carry phages to which they themselves are immune.
Resistance to methicillin and penicillin started in phage group III
Group II: skin infections (impetigo). Group III: production of enterotoxin
How are skin infections caused by Staphylococcus aureus treated: Draining is usually adequate
How are invasive infections caused by Staphylococcus aureus treated: Should be considered a medical emergency. Penicillinase-resistant penicillin or cephalosporin is used. Vancomycin for methycillin resistant strains
How is food poisoning from Staphylococcus aureus treated: Self limited (8 hrs). No treatment necessary unless massive fluid loss occurs
How does Staphylococcus aureus gain resistance to penicillin: A Beta-lactamase encoded by a large molecular weight plasmid. The plasmid may also confer resistance to erythromycin
What encodes resistance to tetracycline, chloramphenicol, and kanamycin in Staphylococcus aureus: Separate low molecular weight plasmids
What is responsible for Staphylococcus aureus strains resistant to methicillin: The mecA gene, which is chromosomally encoded. Vancomycin is used to treat MRSA infections
How is Vancomycin resistance likely acquired in Staphylococcus aureus: Horizontal transfer of vancomycin resistance genes from Enterococcus species
What are the characteristics of coagulase-negative Staphylococcus epidermidis: Normal flora of skin, anterior nares, ear canals
Contaminate specimens rather than cause the infection
Nosocomial infections in patients with catheters and prosthetic devices
Bacteremia in neonatal intensive care units
Slime and polysaccharide antigen (PS/A), easily forms biofilms
What is the most frequent cause of nosocomial infections in patients with catheters and prosthetic devices: Staphylococcus epidermidis
What is the most frequent cause of bacteremia in neonatal intensive care units: Staphylococcus epidermidis
What are the characteristics of coagulase-negative Staphylococcus saprophyticus: Responsible for 10-20% of primary UTIs in females.
Not a major cause of nosocomial infections
Urease may contribute to survival in the bladder and stone formation
What is the definition of a biofilm: Biofilm formation is a process by which microorganisms irreversibly attach to a surface and produce extracellular polymers that facilitate attachment and matrix formation. Formed by many bacteria and they are especially a nuisance in the hospital setting
Why are biofilms medically important: They form on indwelling medical devices and result in bacteremia, urinary tract infections, ect.
They are also associated with increased antibiotic resistance
What are biofilm substrates: Range from very hydrophobic to highly charged and hydrophilic. Rougher and more hydrophobic substances develop biofilms more rapidly.
Substrates in a fluid environment consist mostly of proteins
What are some characteristics of the biofilm structure: Extracellular polymeric substances that consist primarily of polysaccharides. Water channels exist in the matrix
Biofilms act as a filter
How is staphylococcus infection diagnosed in the laboratory: Catalase to distinguish from streptococci and coagulase to determine if S. aureus

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