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The Genus Staphylococcus

• Contains about 40 species and several species have a number of “subspecies” (Bergey’s Manual)

• The two we are concerned about are S. aureus and S. epidermidis

• Gram-positive, non-motile cocci that exist as “grape-like” clusters

• Each cell is almost perfectly spherical and about 1 um in diameter

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Significance of Staphylococcus aureus in Health Settings

From 1980-1989, in a large number of participating hospitals, 26% of all nosocomial infections were caused by coagulase-negative S. aureus strains; 1992-1997, 40% of bacteremias

Causes the most common type of food poisoning reported

Side point: In US, 2,000,000 cases of nosocomial infections per year. Added expenditure to patients estimated at $4.5 billion. Highest rates of infection in: burn, neonatal, and pediatric ICUs.

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Staphylococcus aureus

www.hpa.org.uk/.../staphylo/images/Saureus.jpg

Gram-positive, spherical cellsCells organized in irregular clusters resembling grapesFacultative anaerobe (can ferment)Yellow colonies on most media

“Normal” member of nasal membrane, nasopharynx, skin, perineum, GI tract and genital tract

Causes a variety of infections including:Furuncles (boils), carbuncles, impetigo, toxic epidermal necrolysis, pneumonia, osteomyelitis, meningitis, endocarditis, mastitis, bacteremia, abscesses, food poisoning, enterocolitis, urogenital infections and toxic shock syndrome

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Suppurative infectionsToxic Shock SyndromeSuperficial skin lesionsSerious infectionsDeep seated infectionsFood poisoningNosocomial infections

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Many strains of S. aureus are hemolytic on blood agar plates.

Some strains produce two types of hemolysis: Inner zone caused by toxinOuter zone caused by the toxin

Double zone ofhemolysis

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Portals of Entry

• Hair follicle• Scratch or cut• Needle stick • Surgery scars /sutures• Respiratory tract• GI tract

Spread via blood vesselsSkeletal muscleHeartMeningesKidneyBone

Accumulation of pusTissue necrosisInflammationFibrin clots

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Multifactorial Pathogen• Many virulence factors• Different virulence factors play

either leading or supporting roles in the different diseases– Virulence genes have been

identified– Individual roles of the gene

products have been investigated– Mutants deficient in these traits

are less virulent in animal models– Toxins purified

• Cause symptoms in animal models

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• Surface proteins– Assist in the colonization of host tissues

• Adhesion for collagen– Septic arthritis – infective arthritis– Osteomyelitis – bone or bone marrow infection

• Clumping factor– Fibrin/fibinogen binding protein– Aids in attachment to blot clots & injured tissue

– Attachment proteins• Epithelial tissue• Endothelial tissue

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• Toxins that damage host membranes – Hemolysins

• Almost all strains produce at least one or a combination

• – Binds to a host cell receptor and causes osmotic lysis

• – Sphingomyelinase*– Degrades membranes with sphingomyelin– Most S. aureus strains don’t produce this– Gene for this protein is on bacteriophage

• – Small peptide and its role is unknown

*Sphingomyelin is a lipid found only in some animal cell membranes- particularly in nerve cells

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Toxins that damage host membranes II

• Leukocidin – multicomponent protein toxin that cause pore formation of certain cells (e.g. leukocytes) – may not cause lysis but damages cells

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• Exotoxins– Exfoliatin toxin

• Epidermolytic toxin• Scalded skin syndrome• Causes separation between the layers of skin

– Enterotoxins• Cause diarrhea & vomiting• Responsible for staphylococcal food poisoning• Six different types

– SE-A, B, C, D and G

– Leukotoxin (Leukocidin)• Alter the permeability of the membrane of leukocytes

– Toxic shock syndrome toxin (TSST-1)• Pseudoenterotoxin• Is not emetic• Systemic• Responsible for 75% of TSS• Superantigen

– Large amount of cytokines are released by over stimulated T cells

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• Avoid Phagocytes– Coagulase

• Extracellular protein• Binds & activates prothrombin

– Staphylothrombin– Fibrinogen fibrin

• Hypothesized that clots form and “hide” bacterial cells

– Clotting factor• Fibrinogen binding protein on the bacterial cell surface• Clots form

– Protein A• Binds the FC region of IgG

• Disguises phagocytes• Disrupts opsonization

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• Antiphagocytic Factors– “Microcapsule”

• Thin and requires EM to visualize this capsule• Capsule may only be produced in host

– Leukocidin• Specifically attacks leukocytes• Causes membrane damage but is not

hemolytic

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• Invasions– Assist in the spread of the bacterium

• Kinases– Staphylokinase– Plasmogin activator protein– Lyses fibrin– Hypothesized to aid in bacterial spread

• Hyaluronidase

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• Survival Inside of Phagocytes– Carotenoids

• Golden yellow pigment• Detoxifies reactive oxygen compounds

– Strongly catalase positive• Destroys H2O2 produced by phagocytes

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• Antibiotic resistance– Strains of S. aureus isolated from hospitals are

resistant to antibiotics• Association with nosocomial infections

– There are strains that are resistant to all commonly used antibiotics

– Vancomycin used to be the remaining useful antibiotic

• Vancomycin resistance has been documented• VRSA

– Methicillin resistant S. aureus• MRSA

– Methicillin was developed in the late 1950s as a replacement for penicillin

– MRSA strains were identified in 1961– No longer used clinically

• Have multiple resistance• There are countless MRSA strains • Different degrees of resistance to various antibiotics

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• MRSA infections have increased dramatically– In 1974, infections by MRSA represented

2% of the total number of staph infections

– In 1995, this figure increased to 22%– In 2004, infections by MRSA represented

63%

• MRSA is mainly transmitted to patients via human hands of health care workers– Healthcare-Associated MRSA

• All health care workers must wash their hands with with soap and water or using an alcohol-based hand sanitizer

www.cdc.gov

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VITAL SIGNS: REMEDIES; Tough Policies Against Infection

New York Times, July 27, 2004 Special supplies of clean coats and hand wipes, combined with tough policies on visitors and jewelry, helped a British hospital unit cut hospital-acquired infections drastically, a new study reports. The effort, in the orthopedic unit of Broomfield Hospital in Chelmsford, was described by Dr. Leela C. Biant in a recent article in BMJ, the journal of the British Medical Association. Infections after knee and hip replacement surgery had more than tripled when patients were placed among the general hospital population in 1998, after an orthopedic unit was shut, Dr. Biant wrote. One patient died of an infection with drug resistant staphylococcus, called MRSA. In July 2000, the hospital's orthopedic surgeons set out to seal off a 28-bed ward from infections. The first step was to limit the ward to orthopedic patients, who were tested before surgery for signs of exposure to MRSA; patients who tested positive were restricted to a second, smaller ward, where they were treated with antibiotics as a preventive measure. Along with standard precautions against the spread of infection, nurses were given aprons that they wore at the bedside and then discarded. All staff members washed their hands before they approached a patient and afterward. Doctors wore coats from a supply that was laundered daily. Watches were not allowed on the ward; the only jewelry permitted were wedding rings and small earrings. Visitors were restricted to two at a time for two hours a day only, and were forbidden to sit on the patient's bed. In the year before the policy began, 417 joint replacement operations had produced 43 infections, including nine MRSA cases, Dr. Biant wrote. In the year after its introduction, 488 similar operations produced only 15 infections, none of which were drug resistant, she said.

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Community-Associated MRSA

• MRSA infections acquired by individuals that have not been recently hospitalized or had a medical procedure – Within the past year– Dialysis, surgery, catheters

• Community-Associated MRSA infections present as skin infections including pimples and boils– Occur in otherwise healthy people.

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• Random mutation in chromosomal genes– Selection for resistant strains

• Lateral transfer of resistance genes – R plasmids via conjugation– R genes via transduction

How does antibiotic resistance develop and spread?

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Summary of S. aureus virulence factors

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Carbuncle

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Toxic shock syndrome

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• Toxic shock syndrome (TSS) • Sudden onset of fever, chills, vomiting,

diarrhea, muscle aches and rash • Rapidly progresses to severe and

intractable hypotension and multisystem dysfunction

• Desquamation, especially on the palms and soles – Shedding of the outer layers of the skin – Can occur 1-2 wks after onset of the illness

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• TSS is associated with use of tampons and intravaginal contraceptive devices

• Also occurs as a complication of skin abscesses or surgery

• Risk groups include– Menstruating women– Women using barrier contraceptive devices– Individuals that have undergone nasal

surgery– Individuals with postoperative

staphylococcal wound infections

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S. aureus food poisoning

• Foods: meats, meat products, eggs, and especially mayonnaise-based salads… and raw milk (infection of the udder)

• Caused by enterotoxin that is heat-stable! Cook food, kill bacteria and still get sick…

• Considered to be by many the most common form of food poisoning (probably under-reported)

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Pathogenicity of S. epidermidis

Little known about the pathogenicity of S. epidermidis

- Implicated in plastic foreign body implantations- Cells can bind to fibronectin (deposited

by host onto implants)- Cells can produce a slime resulting in

biofilm formation on implant – not a glycocalyx but a secreted teichoic acid