Clostridium welchii is commonly associated with gas
gangrene. There are live types which are designated as A to E and can be distinguished
by combinations of various toxins they produce. Clostridium welchii of gas
gangrene belongs to type A , and a subgroup within type A is typically associated with Clostridium
welchii food poisoning in man. The other types are associated with diseases in
CHARACTERISTIC OF CLOSTRIDIUM WELCHII
The organism is a relatively large stout Gram-positive bacillus, about 4-6μm by 1μm, with stubby rounded ends, occurring singly or in pairs, and always capsulate when seen in the tissues.
It is non-motile.
Spores are formed under natural conditions, for example in the bowel, but only under special conditions in laboratory media.
It is an anaerobe, but it is not strictly demanding and may grow under microaerophilic conditions.
It grows rapidly in cooked meat broth at 37°C and even more rapidly at temperatures up to 45°C.
It is essentially saccharolytic and is only mildly proteolytic. However, it has various enzymes that enable it to break down cell membranes and connective tissue materials including collagen in animal tissues. On horse blood agar the colonies are large, round, smooth and usually regular with a variable zone of complete haemolysis and sometimes with a wider zone of incomplete haemolysis or darkening. In litmus milk medium this actively saccharolytic organism produces an acid clot that is disrupted by gas production; this is the mechanism of the typical 'stormy clot' reaction.
VIABILITY OF CLOSTRIDIUM WELCHII
Clostridium welchii spores resist the action of the routinely used antiseptics and disinfectants. The spores of classical type-A strains of welchii are only moderately heat-resistant and will not survive boiling for more than a few minutes. The spores of those commonly referred as ‘typical food poisoning strains' and certain type-C strains are markedly heat-resistant and may survive boiling for at least 30 minutes and generally for several hours.
Clostridium welchii differentiated the combinations of various toxic and enzymic factors that they produce. Several of these factors have haemolytic, lethal or necrotizing properties and others have enzymic activity against biological substrates (see below). Various types of a weIchii can be differentiated on the basis of their production of the four major lethal toxins. Type-A strains produce alpha toxin; type-B strains typically produce alpha, beta and epsilon toxins; type-C strains produce alpha and beta toxins; type-D strains produce alpha and epsilon toxins; and type-E strains produce alpha and iota toxins. Neutralization tests may be performed by intracutaneous or intravenous administration of mixtures of toxin and antitoxin to guinea-pig or mice respectively. Epsilon and iota toxins toxin occurs in fully active form in cultures and these prototoxins require to be activated by trypsinization of samples of the culture filtrates prior to neutralization tests.
This toxin is produced by all types of Clostridium welchii but is more noted in type-A strains. It is the lethal toxins of the organism and the main cause of toxaemia that is associated with gas gangrene in human. The alpha toxin is always lethal for laboratory animals and it is necrotizing on intradermal inoculation. It is relatively heat stable, being only 50% inactivated after 5 minutes at 100°C. The toxin is an enzyme—phospholipase (lecithinase C). It can split lipoprotein complexes in serum or egg yolk preparations in the presence of free Calcium or Magnesium ions with a resulting opalescence. The reaction can be inhibited by specific anti-toxin.
The phospholipase also attacks constituents of the membranes of red blood cells of various animals, and the alpha toxin is thereby haemolytic for the red cells of most species except the horse and the goat. The clear zones of haemolysis typically seen around colonies of classical type-A strains of Clostridium welchii grown on horse blood agar are produced by the theta toxin and not by the alpha toxin. With the red cells of the sheep in particular the alpha toxin provides an example of a 'hot-cold' lysin. The alpha toxin similarly damages the membranes of other types of tissue cells and is thus a general cytotoxin.
NAG L ER 'S REACTION.
Several clostridia and other bacterial species are able to produce opalescence in both human serum and egg yolk media due to the production of phospholipases that cause visible precipitates in these media. The reaction was first demonstrated with the alpha toxin of Clostridium welchii and is specifically neutralized by Clostridium welchii alpha antitoxin (but the serologically related phospholipase of Clostridium bifermentans is also inhibited). This reaction has been utilized for the rapid detection of Clostridium welchii in direct plate culture, and allows a serologically controlled identification of the organisms to be made within 20 hours of inoculating the plate from the wound exudate .Further developments of this type of medium have included the incorporation of neomycin sulphate to inhibit aerobic spore forming and coliform bacteria.
Clostridium welchii hence produces a wide range of potentially toxic or aggressin like substances. Cultures of this organism have shown that they possess other enzymic properties. Enzymes are produced, particularly by some type-B strains that destroy blood-group sub-stances. The organism also renders red blood cells inagglutinable by the myxoviruses), by destroying virus receptors at the red cell surface. This is due to a receptor destroying enzyme called neuraminidase which is similar to that of Vibrio cholera. Clostridium welchii is able to renders red blood cells panagglutinable by exposing their T anti-gens so that they lose their specificity and react with any of the ABO antisera. A diffusible haemagglutinin elaborated by Clostridium welchii causes agglutination of the red blood cells of man and most animals. It is produced by some strains after prolonged artificial subculture, but it is not produced by freshly isolated strains. Virulent strains of Clostridium welchii are said to produce an aggressin that has been named 'bursting factor', but this agent has not yet been adequately characterized. The organism also produces a deconjugase enzyme that releases free bile acid from bile salt.
Virulence for animals varies greatly with different strains. Some are markedly pathogenic to guinea pigs by subcutaneous or intramuscular injection of I ml of a 24-hour, toxin-containing culture in cooked-meat broth into the thigh, and the animal may die within 24 hours. A control animal may be protected by a prior injection of Clostridium welchii antitoxin. At necropsy, a spreading inflammatory oedema with gelatinous exudate and gas production is noted in the subcutaneous tissue; necrosis occurs in the underlying muscles which are sodden, friable and pink. The products of growth of the bacillus increase its aggressive and as toxin production occurs during growth, young cultures should be used. The pathogenicity of a strain may be further enhanced by incorporating an equal amount of a sterile 5% solution of calcium chloride in immediately before injection. Pigeons are exceedingly susceptible to experimental inoculation of Clostridium Welchii.
The ill effects of Clostridium welchii gas gangrene are closely related to toxic mechanisms. In 1945, Evans produced indirect evidence that the virulence of Clostridium welchii is primarily related to the production of alpha toxin, but it is debatable whether the toxic products of Clostridium welchii that are recognized invitro studies are the sole agents involved. In 1970, Bullen questioned whether the classical exotoxins have a primary role. There is no doubt that Clostridium welchii is of relatively low infectivity. Nevertheless, it is potentially highly toxigenic and, once established in devitalized tissue, it is fiercely invasive : the organism grows rapidly into surrounding tissue and extends along tissue planes, and the patient rapidly becomes toxaemic and profoundly shocked. Van Heyningen in 1955 postulated the development of a 'muscle toxin' that is, a toxic product resulting from muscle decomposition in vivo: and Collee in 1965 suggested that the organism's neuraminidase could alter host tissue to expose deep T antigens and precipitate a shock mechanism or allow other toxins access to otherwise protected sites.
Clostridium welchii occurs normally in the large intestine of healthy man and animals. It is possible that post gastrectomy diarrhoea is associated in some cases with abnormal growth of Clostridium welchii in the proximal small intestine. The organism can be recovered from human faeces and its spores are ubiquitous in soil, dust and air; they are regularly present on human skin, especially in the region of the perineum, buttocks and thighs. The organism may pass from the bowel to the blood stream in a moribund patient and, multiplying in internal organs after death, produces the small gas cavities sometimes noted (e.g. in the liver) at necropsy. Lowbury and Lilly in 1955 were able show that Clostridium welchii could be isolated from the air of operating theatres. Thus, when gas gangrene occurs after surgical intervention, such as amputation, pinning of a hip fracture or general abdominal surgery, the surgeon may be apt to blame environmental factors such as poor theatre facilities or an inadequate ventilating system, but it is likely that many of these cases arise as a result of endogenous infection .