Staphylococci

Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus saprophyticus

1. Organism – identification
2. Toxins + enzymes – pathogenic mechanisms
3. Host factors
   1. To prevent infection
   2. Predispose to infection
4. Infections
5. Laboratory diagnosis
6. Epidemiology
   1. Spread
   2. Antibiotic resistance
   3. Phage typing

Major source of nosocomial infections: hospital staff may be carrying antibiotic resistant pathogenic strains and infecting susceptible patients. A ubiquitous microorganism carried on body and can survive in air and dust and inanimate objects (fomites). In humans, carriage is on the mucous membranes of the nose (25-30%), in the perineum and the skin of axilla and groin, and in the respiratory and gastrointestinal tracts.

Staphylococci cause disease when host resistance is lowered, commonly infecting wounds but also it is the only organism that causes boils, as the coagulase enzyme allows it to penetrate intact skin. It also causes septicaemia, food poisoning, and infected indwelling foreign devices.

A 1 μm diameter spherical Gram-positive non-motile and non-spore forming catalase positive (most species) cocci that appear as irregular masses (bunch of grapes, hence “staphylo”). They can, therefore, divide along two planes simultaneously and, moreover, may rapidly develop resistance, mainly via plasmid exchange.

Staphylococcus aureus: coagulase positive >> infections of shunts / catheters etc.

Staphylococcus epidermidis (albus) >> universal skin commensal with > 21 species identified.

Staphylococcus saprophyticus >> UTI (Novobiocin resistant, unlike S. epi.)

Also, Staphylococci develop into round, smooth, shiny colonies both aerobically and anaerobically (i.e. particular anaerobes) at 30-37°C nutrient agar after 18 hours incubation.

	Staph. aureus	Staph. Epidermidis
Coagulase	+	–
Mannitol fermentation*	+	–
Pigment	Cream-golden	White

*selective (differential) medium

Staph. Aureus produce acid from mannitol causing a pH change (pink to yellow) and will grown in high [salt] (7.5% NaCl) such as Mannitol Salt agar. Coagulase test identifies the aureus species which are relatively resistant to drying. Consequently, S. aureus survives in cold cured meats like ham.

S. aureus – cell products and pathogenicity

Organism multiplies and spread widely through tissues and produce many extracellular substances.

Toxins

1. Coagulase – changes fibrinogen into fibrin: e.g. in a boil, the organism has produced fibrin and walled itself off; must test plasma (not serum)
2. Haemolysins (a, β, γ, δ) – not important in infectious diseases (only for streptococci)
3. Fibrinolysin (Staphylokinase) – digest fibrin >> fluid effect on boil
4. Leucocidin – kills leucocytes – pathogenic strains do not kill WBC and instead they are phagocytosed and then multiply inside the cell
5. Hyaluronidase – breaks down hyaluronic acid allowing spread through tissues
6. DNAase – hydrolyses DNA
7. Lipase – lipolytic

Individual strains may or may not demonstrate all these features.

Cell Surface Components

• Teichoic acids (cell wall) – strong antigens
• Protein A
  • surface components
  • antiphagocytic
  • interacts with Fc portion of IgG
• Exfoliatin (Phage II strains) – responsible for scalded skin syndrome (SSS)
• Enterotoxin (Phage III strains) – food poisoning by ingestion of toxin >> V/D, CNS stimulation from neural receptors in gut
• Enterotoxin F / Enterotoxin C – toxic shock syndrome (Phage I/III) (may be the same toxins)
  • Abrupt onset of high fever, vomiting, diarrhoea, myalgia, scarlatiniform rash, hypotension, cardiac/renal failure in most severe cases
    • Infections usually confined to one area

Predisposing factors to Staph. aureus infection

1. Injury to normal skin
   1. Trauma
   2. Surgical wounds
   3. Burns
   4. Any primary skin disease (allergic skin reactions)
2. Following viral infection e.g. influenza
   1. often people die due to bacterial pneumonia following flu – die within 18 hours
3. Immunosuppressed
4. Presence of foreign bodies: coagulase negative Staph – indwelling device e.g. catheter
   1. Animal models
      1. Normal: 5 x 10⁶ viable cells needed for infection
      2. Foreign body: 100 viable cells needed for infection
5. Prior antibiotic treatment
6. Others
   1. Diabetes
   2. Alcoholism
   3. CAD
   4. Malignant tumours

Clinical Infections

S. aureus

1. Boil / carbuncle / stye / impetigo (Staph./Strep.) / acne
2. Abscesses of deeper tissues e.g. breasts
3. Infections of lacerations and cuts
4. Blisters of skin, pustules in newborn
5. Infection of surgical wounds / burns
6. Septicaemia / endocarditis (80% mortality)
7. Osteomyelitis – often postoperative can be difficult to eradicate; staph grow in terminal blood vessels of metaphysis of long bones leading to necrosis of bone and chronic suppuration
8. Septic arthritis
9. Pneumonia / empyema
10. Meningitis
11. Enterocolitis / food poisoning (intoxication)
12. Scalded skin syndrome
13. Toxic shock syndrome

Staph epidermidis

1. Infective endocarditis
2. Infection of Spitz-Holter valves
3. Infection of cannulas
4. CAPD (not seen with Staph. Saprophyticus) – others produce adhesin >> allows to stick and grow on indwelling foreign bodies (especially plastics) and produces “slime” (antiphagocytic) covers tissues and resistant to antibiotics and able to stick to fibronectin (specific receptor) and allow penetration

Staph saprophyticus

1. Lipoteichoic acid (in cell wall)
   1. allows to stick to urinary tract cells
   2. UTI (especially sexually active 19-30-year old)
   3. Older women tend to get E. coli / Klebsiella

Diagnosis/Confirmation of Staphylococcal colonisation/infection

	Specimen
Mannitol Salt Agar (MSA)		Blood Agar (BA)
Yellow colonies		Domed, regular, slightly raised, entire, pigmented
Yellow-acid pH fall
Confirm with coagulase test
+/- DNA production

By selecting a specific composition of nutrients, including a high salt content, Mannitol Salt Agar (MSA) acts as a selective and differential medium for the isolation and identification of Staphylococcus aureus by encouraging growth of certain bacteria, including Staph. aureus, while at the same time, and because of, simultaneously inhibiting the growth of other bacteria.

Phage-typing (i.e. using a bacteriophage virus, with a known stable genetic characteristic based on surface receptors, to type the bacteria)

2. Attachment >> S.A.
3. Penetration (of cell wall) >> DNA
4. Replication
5. Maturation
6. Release of new phage particles

25 different phages onto a plate (flooded with strain) – get plaques showing lysis at certain (dark) spots.

Multiresistant Staph. Aureus (MRSA)

• Fucidin (Fu)
• Rifampicin (Ri)
• Vancomycin (VA) – most sensitive

Resistant to other antibiotics

• Penicillin (P)
• Methicillin (Me)
• Erythromycin €
• Tetracycline (T)
• Chloramphenicol ©
• Sulphamethoxazole (S)
• Trimethoprim (Tm)
• Gentamycin (Gm)
• Novobiocin (Nov)

References

Kearney, R. “Staphylococci.” Microbiology lecture – 23/07/1990

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

UW – DC

Version	Date	Modification	Device	Signed
1.0	23/07/1990	Scanned copy of handwritten lecture notes
1.1	25 August 20	MS Word	Dell Desktop	BH