The Staphylococci
The pyogenic cocci ★★★
Most cocci are not associated with human disease. A minority cause human infectious disease, and these are the pathogenic cocci. Because the diseases they cause usually belong to pyogenic infection — pus-generating — they are also called pyogenic cocci.
⭐ The five species — memorise this list
| Gram-positive cocci | Gram-negative cocci |
|---|---|
| Staphylococcus aureus | Neisseria meningitidis |
| Streptococcus pyogenes | Neisseria gonorrhoeae |
| Streptococcus pneumoniae |
Note the qualifier in the lecture's own wording: “only the name of virulence factors and diseases”. Lists, not essays.
- Why are pathogenic cocci also called pyogenic cocci? → The diseases they cause are usually pyogenic — pus-generating
- Name the five pyogenic cocci. → G+: Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae. G−: Neisseria meningitidis, Neisseria gonorrhoeae
Biological characteristics ★★★
The staphylococci exist widely in nature, and humans are the most common carriers — we carry them in the nose, throat and on the skin surface. Three species matter: S. aureus, S. epidermidis and S. saprophyticus.
| Feature | Detail |
|---|---|
| Size | 0.5–1.5 µm in diameter |
| Shape | Round |
| Arrangement | Grape-like clusters |
| Staining | Gram-positive |
| Cultivation | Detail |
|---|---|
| Nutrient requirement | Low — grows very well in basic media |
| Oxygen | Facultative anaerobe |
| Conditions | 37 °C, pH 7.4 |
| Colony | 1–2 mm, circular, smooth, shiny surface, opaque |
| Pigment | Various liposoluble pigments; S. aureus colonies appear golden yellow |
| On blood agar | S. aureus usually forms complete haemolysis (β-haemolysis) |
⭐ The catalase test
Catalase test: positive — 2H₂O₂ → 2H₂O + O₂. Its purpose is to differentiate the staphylococci (positive) from the streptococci (negative).
Resistance to the environment
| Detail | |
|---|---|
| Resistant to | Drying · heat (60 °C for 1 h, 80 °C for 30 min) · high salt concentration (10–15%) — more resistant than other non-spore-forming bacteria |
| Sensitive to | Basic dyes (crystal violet) · antibiotics (penicillin, vancomycin) |
In recent years more and more drug-resistant strains have emerged. MRSA (methicillin-resistant S. aureus) is the most common in nosocomial infection. Nosocomial infection was defined in Unit 6 §6 — hospital-acquired — and MRSA is its emblem.
The salt tolerance is not a laboratory curiosity either. Tolerating 10–15% NaCl is precisely what allows S. aureus to live on salty human skin, where sweat concentrates as it evaporates. It is also the basis of mannitol-salt agar, the selective medium that grows staphylococci while suppressing almost everything else — and, since S. aureus ferments mannitol and the other two do not, identifies the pathogen in the same step.
One environmental property; one ecological niche and one culture medium.
- Describe staphylococcal morphology. → Gram-positive, round, 0.5–1.5 µm, in grape-like clusters
- Culture characteristics? → Low nutrient requirement, facultative anaerobe, 37 °C, pH 7.4; 1–2 mm circular smooth shiny opaque colonies; S. aureus golden yellow with β-haemolysis
- What does the catalase test distinguish? → Staphylococci (positive) from streptococci (negative)
- What is S. aureus resistant to? → Drying, heat (60 °C/1 h, 80 °C/30 min), 10–15% salt — more than other non-spore formers
- What is MRSA and why does it matter? → Methicillin-resistant S. aureus, the commonest organism in nosocomial infection
⭐ SPA — Staphylococcal Protein A ★★★
A surface protein of Staphylococcus aureus, covalently bound to the peptidoglycan, which combines non-specifically with the Fc fragment of IgG.
Functions
| As a virulence factor | As a reagent |
|---|---|
| Anti-phagocytosis | Coagglutination test |
| Damages platelets | |
| Activates B cells | |
| Induces hypersensitivity and inflammation |
Then the four functions for the remaining marks: it is a virulence factor — anti-phagocytic, it damages platelets, activates B cells, and induces hypersensitivity and inflammation. And its laboratory use: the coagglutination test.
Do not stop at “a surface protein” — the Fc-binding and the anti-phagocytic consequence are what the four marks are for.
An antibody has two ends. The Fab arms grip the antigen; the Fc stem is the handle that a phagocyte's Fc receptor grabs in order to eat the target. That handle is what opsonisation means.
SPA binds the Fc end. So antibody coats S. aureus the wrong way round — Fab arms pointing outward into space, Fc stems buried against the bacterium. There is no handle left for the neutrophil to grab.
Hence anti-phagocytosis, and hence, in Unit 5's framework, mechanisms for escaping elimination by host defences.
The coagglutination test is the same property turned into a reagent: coat SPA-bearing staphylococci with an antiserum, the Fc ends stick to the bacteria and the Fab arms all point outward — a ready-made particle that clumps visibly the moment it meets its antigen. The bacterium's defence becomes the laboratory's tool.
- Define SPA. → A surface protein of S. aureus, covalently bound to the peptidoglycan, binding non-specifically to the Fc fragment of IgG
- Name its four functions as a virulence factor. → Anti-phagocytosis · damages platelets · activates B cells · induces hypersensitivity and inflammation
- What is its laboratory use? → The coagglutination test
- Why does Fc binding block phagocytosis? → It coats the organism with antibody the wrong way round, leaving no Fc handle for the phagocyte's Fc receptor
The three species compared ★★★
Classification is based on the pigments and chemical reactions — more than 30 species. Three matter clinically, and this table separates them.
| Property | S. aureus | S. epidermidis | S. saprophyticus |
|---|---|---|---|
| Pigment | Gold-yellow | White | Citrine |
| Coagulase | + | − | − |
| Mannitol | + | − | − |
| Thermostable nuclease | + | − | − |
| α-haemolysin | + | − | − |
| SPA | + | − | − |
| Pathogenicity | Potent | Weak | Weak |
There is a second, simpler classification based on coagulase: coagulase-positive staphylococci — S. aureus; and coagulase-negative staphylococci — S. epidermidis and S. saprophyticus.
- Name the three species and their pigments. → S. aureus gold-yellow · S. epidermidis white · S. saprophyticus citrine
- Which five tests are positive only in S. aureus? → Coagulase, mannitol, thermostable nuclease, α-haemolysin, SPA
- What is the coagulase-based classification? → Coagulase-positive (S. aureus) vs coagulase-negative (S. epidermidis, S. saprophyticus)
⭐ Virulence factors of S. aureus ★★★
Half of the 2019 seven-mark question. The lecture groups them in three: invasiveness — surface structures and enzymes — and toxins.
| Group | Factors |
|---|---|
| Invasiveness — surface structure | SPA, and others |
| Invasiveness — enzymes | Coagulase · hyaluronidase · thermostable nuclease · staphylokinase |
| Toxins (exotoxins) | Staphylolysin · leukocidin · staphylococcal enterotoxin · toxic shock syndrome toxin-1 (TSST-1) · exfoliatin |
⭐ Coagulase — the most examinable enzyme
An enzyme that converts soluble fibrinogen into fibrin, causing coagulation of blood or plasma.
| Detail | |
|---|---|
| Classification | Free coagulase — an extracellular enzyme · bound coagulase — bound to the surface of cells |
| Roles | Fibrin-coated bacteria and bacterial clumps inhibit phagocytosis and reduce the damage from antibacterial substances in the body · form a limited abscess |
| Laboratory use | The major criterion for differentiating potentially pathogenic staphylococci |
The other enzymes
| Enzyme | Action |
|---|---|
| Hyaluronidase | Hydrolyses hyaluronic acid |
| Thermostable nuclease | Degrades DNA |
| Staphylokinase | Dissolves fibrin clots |
The five toxins
| Toxin | Role |
|---|---|
| Staphylolysin | Damages host cell membranes; cytotoxic to phagocytes and tissue cells. Several kinds — α-lysin is the main pathogenic substance and causes the haemolysis ring around the colony; also β, γ and δ |
| Leukocidin | Lyses neutrophils and macrophages by damaging cell membranes |
| Staphylococcal enterotoxin | Protein, heat-stable (100 °C for 30 min), resistant to proteolytic enzymes. Causes staphylococcal food poisoning and colitis with severe vomiting and diarrhoea, by acting on the CNS vomiting-control centre. Acts as a superantigen |
| TSST-1 | Causes toxic shock syndrome — a multisystem disease with high fever, shock and multi-organ failure. Increases the host's sensitivity to endotoxin. Death from profound shock, respiratory failure and DIC |
| Exfoliatin | Causes blistering of the skin, associated with SSSS (staphylococcal scalded skin syndrome) in infants and young children. Erythema → denuding of skin. Mortality is low |
One enzyme makes fibrin; another dissolves it. That is not sloppiness, it is sequence.
Coagulase first. Clotting the plasma around itself walls the organism off from neutrophils and from antibiotics, and the result is the characteristic staphylococcal lesion: a localised abscess with thick pus. This is why staphylococcal infection is a boil, sharply demarcated, rather than the spreading cellulitis of a streptococcus — which, as Unit 9 shows, makes streptokinase and hyaluronidase instead.
Staphylokinase later. Having built the abscess and multiplied inside it, the organism dissolves the fibrin and escapes to seed elsewhere — osteomyelitis, endocarditis, septicaemia.
Build the fortress, then break out of it. Two enzymes, one strategy.
- Name the three groups of S. aureus virulence factors. → Surface structures (SPA) · enzymes · toxins
- Name the four enzymes. → Coagulase, hyaluronidase, thermostable nuclease, staphylokinase
- Define coagulase and state its roles. → Converts soluble fibrinogen to fibrin; free and bound forms; fibrin coating inhibits phagocytosis and forms a limited abscess; the major criterion for identifying pathogenic staphylococci
- Name the five toxins. → Staphylolysin, leukocidin, enterotoxin, TSST-1, exfoliatin
- Which toxin is heat-stable, and what does it cause? → Staphylococcal enterotoxin (100 °C for 30 min); food poisoning and colitis with severe vomiting and diarrhoea, by acting on the CNS vomiting centre; a superantigen
- What does TSST-1 do? → Causes toxic shock syndrome — high fever, shock, multi-organ failure; increases sensitivity to endotoxin; death from shock, respiratory failure and DIC
⭐ Diseases of S. aureus ★★★
The other half of the 2019 question. The lecture divides them in two, and that division is itself worth a mark.
| Group | Diseases |
|---|---|
| Invasive / pyogenic infection — characterised by abscess formation | Local (skin) infection — hair folliculitis · boil (furuncle) · carbuncle · impetigo. Characteristics of the local abscess: thick pus, limited local area — related to the action of coagulases Organ infection — pneumonia, meningitis Systemic infection — septicaemia |
| Toxin-associated disease | Food poisoning (enterotoxin) · TSS — toxic shock syndrome (TSST-1) · SSSS — staphylococcal scalded skin syndrome (exfoliatin) |
Virulence factors. Surface structure — SPA (binds IgG Fc, anti-phagocytic). Enzymes — coagulase (fibrinogen → fibrin, anti-phagocytic, localises the abscess), hyaluronidase, thermostable nuclease, staphylokinase. Toxins — staphylolysin (α-lysin the main one), leukocidin, enterotoxin, TSST-1, exfoliatin.
Diseases, in two groups. Invasive/pyogenic, with abscess formation — local: folliculitis, boil, carbuncle, impetigo; organ: pneumonia, meningitis; systemic: septicaemia. Toxin-associated — food poisoning (enterotoxin), toxic shock syndrome (TSST-1), scalded skin syndrome (exfoliatin).
Pair each toxin-associated disease with its toxin in brackets. That single habit shows the examiner you know which factor causes which disease, rather than having memorised two separate lists.
- How are S. aureus diseases divided? → Invasive/pyogenic (abscess-forming) and toxin-associated
- Name four local skin infections. → Hair folliculitis, boil (furuncle), carbuncle, impetigo
- What characterises the local abscess, and why? → Thick pus in a limited local area — due to coagulase
- Name the three toxin-associated diseases and their toxins. → Food poisoning (enterotoxin), toxic shock syndrome (TSST-1), scalded skin syndrome (exfoliatin)
Laboratory diagnosis and control ★★
| Step | Detail |
|---|---|
| Specimen | Direct smear and Gram stain |
| Isolation | Blood agar — gold-yellow pigmentation, β-haemolysis |
| Identification | Coagulase test · catalase test |
| Then | Antibiotic susceptibility tests |
| Control | Detail |
|---|---|
| Treatment | Antibiotics, based on the drug sensitivity test |
| Prevention | Proper personal hygiene, sanitary procedures and aseptic measures · identifying and treating carriers as soon as possible |
- Outline the laboratory diagnosis of S. aureus. → Direct smear and Gram stain → isolation on blood agar (gold-yellow pigment, β-haemolysis) → coagulase and catalase tests → antibiotic susceptibility test
- How is it controlled? → Antibiotics guided by sensitivity testing; personal hygiene, sanitary and aseptic measures; identifying and treating carriers early
Revision
The two tests that decide everything
| Test | Separates |
|---|---|
| Catalase | Staphylococci (+) from streptococci (−) |
| Coagulase | S. aureus (+) from the coagulase-negative staphylococci (−) |
The whole unit on one screen
| Question | Answer |
|---|---|
| Five pyogenic cocci? | S. aureus · S. pyogenes · S. pneumoniae · N. meningitidis · N. gonorrhoeae |
| Morphology? | G+, round, 0.5–1.5 µm, grape-like clusters |
| Colony? | 1–2 mm, circular, smooth, shiny, opaque; gold-yellow, β-haemolysis |
| Resistance? | Drying · 60 °C/1 h, 80 °C/30 min · 10–15% salt |
| ⭐ SPA? | Surface protein bound to peptidoglycan, binds IgG Fc; anti-phagocytic, damages platelets, activates B cells, induces hypersensitivity; coagglutination test |
| Enzymes? | Coagulase · hyaluronidase · thermostable nuclease · staphylokinase |
| Toxins? | Staphylolysin (α main) · leukocidin · enterotoxin · TSST-1 · exfoliatin |
| Pyogenic diseases? | Folliculitis, boil, carbuncle, impetigo · pneumonia, meningitis · septicaemia |
| Toxin diseases? | Food poisoning · TSS · SSSS |
| MRSA? | Methicillin-resistant S. aureus — commonest in nosocomial infection |
- Answer the 2019 question in full. → Virulence: SPA; coagulase, hyaluronidase, thermostable nuclease, staphylokinase; staphylolysin, leukocidin, enterotoxin, TSST-1, exfoliatin. Diseases: pyogenic (folliculitis, boil, carbuncle, impetigo; pneumonia, meningitis; septicaemia) and toxin-associated (food poisoning, TSS, SSSS)
- Define SPA for 4 marks. → Surface protein of S. aureus covalently bound to peptidoglycan, binding non-specifically to the Fc fragment of IgG; anti-phagocytic, damages platelets, activates B cells, induces hypersensitivity; used in the coagglutination test
- Which two tests identify S. aureus? → Catalase (staph vs strep) and coagulase (S. aureus vs the coagulase-negative species)
- Why does S. aureus form a localised abscess? → Coagulase converts fibrinogen to fibrin, walling the lesion off and inhibiting phagocytosis