Bacteriology - Wound Infections (Clostridia, Staphylococci, Pseudomonas) Flashcards
Staphylococcus, pseudomonas, clostridium.
Outline the key features of bacterial cells.
- The cytoplasm (bound peripherally by cytoplasmic membrane (a conventional phospholipid bilayer)).
- Outside, and closely covering this, lies the rigid, supporting cell wall, which is porous and relatively permeable.
- Cell division occurs by the development of constrictions mediated by the assembly of an actin-like protein, FtsZ. The constrictions proceed from the periphery inwards and, in some cases, produce a transverse cell wall known as a septum or cross-wall.
- The cytoplasm is a predominantly aqueous environment packed with ribosomes.
- The cytoplasmic contents are not normally visible by light microscopy. Bacteria have an extensive cytoskeletal network which, in different genera, include structures corresponding to eukaryotic actin, tubulin and intermediate filaments (e.g. MreB, FtsZ and crescentin, respectively). The importance of these is emerging in determining cell shape, division and spore formation (see below). Antimicrobials target these functions.
- Some larger structures such as spores or inclusion bodies of storage products such as volutin (polyphosphate), lipid (e.g. poly-β-hydroxyalkanoate or triacylglycerol), glycogen or starch occur in some species under specific growth conditions.
- Outside the cell wall there may be a protective gelatinous covering layer called a capsule or, when it is too thin to be resolved with the light microscope, a microcapsule.
- Some bacteria bear, protruding outwards from the cell wall, one or more kinds of protein-based filamentous appendages called flagella, which are organs of locomotion, and hair-like structures termed fimbriae or pili, which, via specific receptor–ligand interactions at their tip, mediate adhesion.
Outline the shapes and characteristic groupings of bacteria.
Bacterial cells may have two basic shapes, spherical (coccus) or rod shaped (bacillus); the rod-shaped bacteria show variants that are comma shaped (vibrio), spiral (spirillum and spirochaete) or filamentous
Outline the bacterial cell wall.
It is 10–25 nm thick, strong and relatively rigid, though with some elasticity, and openly porous, being freely permeable to solute molecules smaller than 10 kDa in mass and 1 nm in diameter.
Gram +ve cells have a thick 20-80 nm peptidoglycan layer, whereas gram -ve cells have a thin 5-10 nm peptidoglycan layer.
A: gram +ve, B: gram -ve
Outline Staphylococci.
Staphylococci are commonly found on the skin of healthy individuals. Staph. aureus is present in the nose of 30% of healthy people but can cause infections where there is lowered host resistance (e.g. damaged skin).
- Facultative anaerobe (aerobic, but can switch to anaerobic), non-sporulating, non-motile, Gram positive cocci.
- 27 species, but three are medically important; S. aureus, S. epidermidis, and S. saprophyticus.
Name some medically important Staphylococci.
S. aureus (important human pathogen), S. epidermidis (normal flora, but disease under certain circumstances), and S. saprophyticus (UTIs).
Outline Staphylococcus aureus.
Staphylococcus aureus causes a wide range of major and minor infections in man and animals and is characterized by its ability to clot blood plasma by the action of the enzyme coagulase.
Gram-positive coccus about 1 μm in diameter. The cocci are usually arranged in grape-like clusters. The organisms are non-sporing, non-motile and usually non-capsulate.
When grown on many types of agar (e.g. sheeps blood agar) for 24 h at 37°C, individual colonies are circular, 2–3 mm in diameter, with a smooth, shiny surface; colonies appear opaque and are often pigmented (golden-yellow, hence the ‘aureus’).
Outline the diagnostic features of Staphylococcus aureus. [3]
- Production of an extracellular enzyme, coagulase, which converts plasma fibrinogen into fibrin, aided by an activator present in plasma.
- Production of thermostable nucleases that break down DNA.
- Production of a surface-associated protein known as clumping factor (or bound coagulase) that reacts with fibrinogen.
Name some infections that can be caused by Staphylococcus aureus.
Name some toxins produced by Staphylococcus aureus.
- Enterotoxins, types A–E, G, H, I and J, are commonly produced by up to 65% of strains of Staph. aureus. When ingested as preformed toxins in contaminated food, microgram amounts of toxin can, within a few hours, induce the symptoms of staphylococcal food poisoning: nausea, vomiting and diarrhoea.
- Two kinds of epidermolytic toxin (types A and B) are commonly produced by strains that cause blistering diseases. These toxins induce intraepidermal blisters at the granular cell layer. Such blisters range in severity from the trivial to the distended blisters of pemphigus neonatorum. The most dramatic manifestation of epidermolytic toxin is the scalded skin syndrome in small children.
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In general, healthy people don’t get Staph infections. Name some of the risk factors.
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Skin Disease
- Increased colonization
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Trauma
- Expose binding sites
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Viral Respiratory Tract Infection (Influenza)
- Expose binding sites
- Decreased clearance
- Neoplasia (benign growth)
- Diabetes
- Broad Spectrum Antibiotic Therapy
Staphylococcus aureus can cause a variety of infections. Outline the main differences in folliculitis, furuncles, and carbuncles.
- Folliculitis: inflammation of hair follicles from infection.
- Furuncles: often starts as folliculitis, firm, tender, red nodule (painful), drains spontaneously.
- Carbuncles: larger than furuncles, extends to subcutaneous fat, interconnected.
Name Staph infections that can occur in the different layers of skin.
- Impetigo: superficial infection, also caused by Strep. pyogenes, more common in children and in hot weather, infection of minor trauma, starts in vesicles.
- Cellulitis: an acute spreading infection, involves both the skin and subcutaneous tissues, infection of skin trauma, warm and erethematous.
Outline food poisoning caused by Staphylococcus.
- Caused by toxigenic strains of Staph. aureus (produces enterotoxin B).
- Person-to-person transmission.
- Commonly in custard-filled goods, canned food, potato salad, ice-cream. Food appears normal in colour, smell, and taste.
- Incubation period 2-6 hours.
- Salivation, nausea, vomiting, abdominal cramps, watery diarrhoea.
- Self limited, symptoms disappear in 8 hours.
Outline MRSA.
MRSA produces a penicillin binding protein 2a (mediated through the mecA gene), which is carried on the staphylococcal cassette chromosome mec (SCCmec) of which there are at least six different types recognized, and this results in resistance to all beta-lactam antibiotics.
Glycopeptides (vancomycin or teicoplanin) are the agents of choice in the treatment of systemic infection with MRSA, but these agents are relatively expensive and may be toxic.
How are Staph infections treated?
Staph. aureus and other staphylococci are inherently susceptible to many antimicrobial agents. About 90% of strains found in hospitals are now resistant to benzylpenicillin due to the production of the enzyme penicillinase, a β-lactamase that opens the β-lactam ring. Methicillin, oxacillin, cloxacillin and flucloxacillin, are stable to the enzyme. Cephalosporins and β-lactamase inhibitors are also stable to penicillinase
Outline Pseudomonads.
- Pseudomonads are aerobic, saprophytic (feeds on decaying matter) and innately resistant bacteria causing opportunist infections. The most important pseudomonad species responsible for human infections are Pseudomonas aeruginosa, Burkholderia pseudomallei (tropical) and members of the Burkholderia cepaciacomplex (immunocompromised).
- Pseudomonas aeruginosa is an important cause of hospital-acquired infections, especially in intensive care units and in neutropenic (low neutrophil count) patients. Infections range from topical to systemic and may be trivial or life-threatening. Common among burn victims.
- Gram -ve, rod shaped
- Virulence factors: adhesins, toxins, polysaccharide capsule.
- The bacteria kills cells, destroys tissue, and triggers shock.
Outline Pseudomonas aeruginosa.
Ps. aeruginosa is a non-sporing, non-capsulate, non-fastidious (doesn’t need specific growth media) Gram-negative bacillus; it is usually motile by virtue of one or two polar flagella. It is a strict aerobe but can grow anaerobically if nitrate is available as a terminal electron acceptor. The organism grows readily on a wide variety of culture media, over a wide temperature range, and emits a sweet grape-like odour that is easily recognized.
It is rarely part of the normal microbiome, and can cause infection throughout the body once inside.
How are Pseudomonas infections diagnosed and treated?
- Material should be cultured on a selective medium that enhances the production of pyocyanin to aid detection.
- Various reliable polymerase chain reaction (PCR) assays are also available.
- Tests for serum antibodies against pseudomonas antigens have no place in diagnosis except in patients with cystic fibrosis or chronic obstructive pulmonary disease.
- Treatment of Ps. aeruginosa infection remains challenging as most isolates are intrinsically resistant to commonly used antimicrobial agents.
- Aminoglycosides (gentamicin, amikacin and tobramycin) and β-lactam compounds (piperacillin, ticarcillin, ceftazidime and carbapenems) are often used, alone or in combination.
Outline Clostridium spp.
- The clostridia are Gram-positive spore-bearing anaerobic bacilli. Most species are saprophytes.
- All clostridial infections are characterized by toxin production by the infecting species.
- Clostridia are biochemically active, frequently possessing both saccharolytic and proteolytic properties, although in varying degrees. Many species are highly toxigenic.
- The major toxins produced by the species are neurotoxins affecting nervous tissue, histotoxins affecting soft tissue and enterotoxins affecting the gut.
- All produce spores, which enable the organisms to survive in adverse conditions, for example in soil and dust and on skin.
What Clostridium spp. are associated with human disease?
Pathogenic species include:
- C. perfringens, C. septicum and C. novyi, the causes of gas gangrene and other infections. C. perfringens is also associated with a form of food poisoning.
- C. tetani, the cause of tetanus.
- C. botulinum, the cause of botulism.
- C. difficile, the cause of pseudomembranous colitis and antibiotic-associated diarrhoea.
What types of spores do Clostridium spp. have?
Most Clostridium spp., including C. perfringens and C. botulinum, have ovoid subterminal (OST) spores
C. tetani have round terminal (RT) spores
Outline Clostridium perfringens.
C. perfringens is a relatively large Gram-positive bacillus with blunt ends. It is capsulate and non-motile.
C. perfringens is the most common cause of gas gangrene, although various other species of clostridia are occasionally implicated, either alone or in combination.
The disease is characterized by rapidly spreading oedema, myositis, necrosis of tissues, gas production and profound toxaemia occurring as a complication of wound infection.
When ingested in large numbers some strains of C. perfringens produce an enterotoxin in the gut, causing diarrhoea and other symptoms of food poisoning
How are Clostridium perfringens species distinguished?
Clostridium perfringens produces alpha toxin, which is a lecithinase. On a medium containing egg yolk (lecithin), enzyme activity can be detected as opacity around the line of growth (right). Known as the Nagler reaction.
If anti-alpha toxin is applied to the surface of the plate, the action of the toxin is inhibited (left). Used to confirm the identity of a clostridial isolate.
Outline Clostridium tetani.
- The tetanus bacillus is a motile, straight, slender, Gram-positive rod.
- A fully developed terminal spore gives the organism the appearance of a drumstick with a large round end. It is an obligate anaerobe (dies in O2).
- As in gas gangrene, germination of spores and their outgrowth depend upon reduced oxygen in devitalised tissue and non-viable material in a wound. The tetanus bacillus remains strictly localized, but tetanus toxin is elaborated and diffuses.
- C. tetani produces an oxygen-labile haemolysin (tetanolysin), but the organism’s neurotoxin (tetanospasmin) is the essential pathogenic product. Strains vary in their toxigenicity; some are highly toxigenic. Most strains produce demonstrable toxin after culture in broth for a few days.
How does Clostridium tetani affect the nervous system?
- The tetanus bacilli stay at the site of the initial infection and the toxin diffuses to affect the relevant level of the spinal cord (local tetanus) and then to affect the entire system (generalized tetanus).
- The toxin is absorbed from the site of its production, but may be delivered via the blood to all nerves in the body.
- The heavy chain mediates attachment to gangliosides and the toxin is internalised. It is then moved from the peripheral to the central nervous system by retrograde axonal transport and trans-synaptic spread. The tendency for the first signs of human tetanus to be in the head and neck is attributed to the shorter length of the cranial nerves.
- Once the entire toxin molecule has been internalised into presynaptic cells, the light chain is released and affects the membrane of synaptic vesicles. This prevents the release of the neurotransmitter γ-aminobutyric acid. Motor neurones are left under no inhibitory control and undergo sustained excitatory discharge, causing the characteristic motor spasms of tetanus.
- The toxin exerts its effects on the spinal cord, brainstem, peripheral nerves, at neuromuscular junctions and directly on muscles.
What are the clinical forms of tetanus?
- Tetanus neonatorum follows infection of the umbilical wound of newborn infants
- Generalised tetanus
- Localised/wound tetanus
- Cephalic tetanus
- Gradual onset, usually starting with some stiffness and perhaps pain in or near a recent wound. In some cases the initial complaint may be of stiffness of the jaw (lockjaw). The period between injury and the first signs is usually about 10–14 days, but there is a considerable range. A severe case with a relatively poor prognosis shows rapid progression from the first signs to the development of generalised spasms. Sweating, tachycardia and arrhythmia, and swings in blood pressure, reflect sympathetic stimulation.
How is tetanus diagnosed and treated?
Diagnosis is based off of clinical presentation, microscoscopy is not definitive.
Direct culture of unheated material on blood agar incubated anaerobically is often the best method of detecting C. tetani.
The patient is given 10 000 units of human tetanus immunoglobulin (HTIG) in saline by slow intravenous infusion. Full wound exploration and debridement (removal of damaged tissue) is arranged, and the wound is cleansed and left open with a loose pack. Penicillin or metronidazole is given for as long as considered necessary to ensure that bacterial growth and toxin production are stopped. The antitoxin and antibiotics are given immediately, and preferably before surgical excision.
Prevention through vaccination, boosters every 10 years.
Outline Clostridium botulinum.
- C. botulinum is a strictly anaerobic Gram-positive bacillus. It is motile and has spores that are oval and subterminal. It is a widely distributed saprophyte found in soil.
- The widespread occurrence of C. botulinum in nature, its ability to produce a potent neurotoxin in food, and the resistance of its spores to inactivation combine to make it a formidable pathogen.
- Botulism: rare but very severe form of food poisoning in which symptoms are neurological rather than intestinal.
Describe botulism.
Botulism is a severe, often fatal, form of food poisoning characterised by pronounced neurotoxic effects. Botulinum toxins are among the most poisonous natural substances known. Seven main types of C. botulinum, designated A–G, produce antigenically distinct toxins with pharmacologically identical actions. All types can cause human disease, but types A, B and E are most common.
The disease has been linked to a wide range of foods, including preserved hams, large sausages of the salami type, home-preserved meats and vegetables, canned products such as fish, liver paste, and even hazelnut purée and honey.
The pre-formed toxin in the food is absorbed from the intestinal tract. Although it is protein, intestinal proteolytic enzymes do not inactivate it. After absorption, botulinum toxin binds irreversibly to the presynaptic nerve endings of the peripheral nervous system and cranial nerves, where it inhibits acetylcholine release.
What are the clinical forms of botulism?
- The period between ingestion of the toxin and the appearance of signs and symptoms is usually 1–2 days, but it may be much longer. There may be initial nausea and vomiting. The oculomotor muscles are affected, and the patient may have diplopia (double vision) and drooping eyelids with a squint. There may be vertigo and blurred vision.
- There is progressive descending motor loss with flaccid paralysis.
- Wound botulism
- Infant botulism: The ‘floppy child syndrome’ describes a young child, usually less than 6 months old, with flaccid paralysis
