Bacteriology

CLOSTRIDIUM BOTULINUM

Written by MicroDok

Clostridium botulinum is a Gram-positive, strict-anaerobic, motile, pleomorphic, catalase-negative, endospore-forming bacillus (rod) that is ubiquitously found in the soil. The endopores of C. botulinum are sub-terminally placed or located on the bacterium and they are oval in shape. C. botulinum is found in the genus Clostridium and class Clostridia; and members are known to cause food spoilage, gas gangrene, botulism and tetanus. Clostridium species are able to ferment a wide variety of organic compounds, and they produce butyric acid, acetic acid, butanol and acetone, and large amounts of gas (CO2 and H2) as end products during the fermentation of sugars. The ability of Clostridium species to produce gas under anaerobic conditions is the reason why most canned foods contaminated by the pathogen are swollen. It is the causative agent of botulism, a non-communicable disease and a type of food poisoning caused by the exotoxin produced by C. botulinum.

Microscopical illustration of the Gram-positive, rod-shaped and spore-forming morphology of C. botulinum. Notice the terminally-placed spore of the organism.

PATHOGENESIS OF CLOSTRIDIUM BOTULINUM INFECTION

Food-borne botulism or C. botulinum infection in humans occurs following the ingestion of food containing preformed exotoxins (neurotoxin) formed by the pathogen. When home-made canned foods are not properly or well heated to kill the contaminating endopores of C. botulinum, food-intoxication is bound to occur in the individual. Most foods that are eaten almost raw i.e. without cooking are mainly the once with contaminating endospores of C. botulinum. Once these foods become contaminated by the pathogen, C. botulinum grows anaerobically to produce its exotoxin. C. botulinum produces seven types of exotoxins (A – G), but human infections are basically caused by the toxigenic types: A, B, E and F. The environment (soil) is the main source of infection with C. botulinum. After ingestion of the exotoxins, the toxin is absorbed by the gastrointestinal tract (GIT) from where it is transported to the peripheral nervous system. Notably, C. botulinum neurotoxin blocks neuroexocytosis of vesicles containing acetylcholine (a neurotransmitter). This phenomenon goes on to prevent nervous stimulation of the host’s muscles that leads to a flaccid paralysis that resembles that caused by tetanus infection but without muscle contractions. Spasm is used for tetanus infection while paralysis is used to describe botulism.

LABORATORY DIAGNOSIS OF CLOSTRIDIUM BOTULINUM INFECTION

The laboratory diagnosis of C. botulinum food-intoxication (botulism) is by the identification of the pathogen in contaminated food samples, intestinal contents and detection of the organism’s toxin in blood (serum) samples of infected individuals. Stool, vomitus and tissue samples can also be obtained from the patients depending on the type of botulism being investigated. Serum is irrelevant in detecting infant botulism in neonates. The causative agent of botulism is not usually cultured but suspect food samples containing C. botulinum can be cultured in specialized selective media such as the Robertson’s cooked meat medium (RCMM) and the lactose egg yolk milk agar under anaerobic conditions at 35oC for about 2-5 days. Identification of the spores of C. botulinum in a stained preparation can also be employed in the presumptive detection of the pathogen.

TREATMENT OF CLOSTRIDIUM BOTULINUM INFECTION

Supportive care is mainly needed to manage food intoxication caused by C. botulinum. However, a polyvalent antitoxin (of equine origin) is administered intravenously with carefulness. But this antitoxin/antiserum is rarely used in some clinical conditions due to the allergic reactions it sparks in patients (especially infants). Respiratory failures (or pharyngeal paralysis) can cause mortality thus proper ventilation should be maintained in all cases using an artificial breathing apparatus to supply oxygen. On the contrary, most cases of infant botulism are self limiting, and neonates recover from the infection when given proper supportive care. Antibiotics (e.g. amoxicillin) can be administered in cases of wound botulism.

PREVENTION AND CONTROL OF CLOSTRIDIUM BOTULINUM INFECTION

Though food intoxication caused by C. botulinum is an uncommon disease, the prevention of botulism is important and lies in the proper cooking of our foods. Since adequate pressure cooking and autoclaving kills the endopores of C. botulinum, these measures must always be applied in the processing of our foods.

REFERENCES

Prescott L.M., Harley J.P and Klein D.A (2005). Microbiology. 6th ed. McGraw Hill Publishers, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Balows A, Hausler W, Herrmann K.L, Isenberg H.D and Shadomy H.J (1991). Manual of clinical microbiology. 5th ed. American Society of Microbiology Press, USA.

Barrett   J.T (1998).  Microbiology and Immunology Concepts.  Philadelphia,   PA:  Lippincott-Raven Publishers. USA.

Basic laboratory procedures in clinical bacteriology. World Health Organization (WHO), 1991. Available from WHO publications, 1211 Geneva, 27-Switzerland.

Murray P.R, Baron E.J, Jorgensen J.H., Pfaller M.A and Yolken R.H (2003). Manual of Clinical Microbiology. 8th edition. Volume 1. American Society of Microbiology (ASM) Press, Washington, D.C, U.S.A.

Murray P.R, Baron E.J, Jorgensen J.H., Pfaller M.A and Yolken R.H (2003). Manual of Clinical Microbiology. 8th edition. Volume 2. American Society of Microbiology (ASM) Press, Washington, D.C, U.S.A.

Murray P.R., Rosenthal K.S., Kobayashi G.S., Pfaller M. A. (2002). Medical Microbiology. 4th edition. Mosby Publishers, Chile.

 

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