Clostridia

Clostridia are Gram-positive, rod-like, strictly anaerobic, sporogenous bacteria. Clostridial spores are able to withstand aerobic and extreme conditions (e.g. temperatures of more than 100 °C). This facilitates the colonization of diverse habitats:

Clostridia are found in soil and surface water and can colonise the digestive tract of humans and higher animals as commensals. In addition to non-pathogenic species, the Clostridia also comprise pathogens causing tetanus (Clostridium tetani), blackleg (Clostridium chauvoei) or gas gangrene (e.g. Clostridium perfringens or Clostridium septicum). This pathogenicity is usually associated with the ability to produce toxins.

Clostridium (C.) perfringens is a toxin producing species found as a wound pathogen and particularly frequently as a causative agent in foodborne intoxication.  C. perfringens strains are classified since 2018 according to Root et al. into types A - G (toxovars) based on their toxin producing ability, with certain toxin types being associated with diseases in specific host species. For example, necrotic enteritis in chickens is caused by C. perfingens type G and human food intoxications are caused by C. perfringens type F (formerly type A). In addition to the six toxins alpha, beta, epsilon, iota, enterotoxin und NetB - which are important for the toxinotyping - C. perfringens possesses a further 16 known variants of the so-called minor toxins.

The bacterium grows like other Clostridium species primarily under anaerobic conditions, but it is also able to multiply in the presence of oxygen, albeit at a reduced rate. By heating food to a core temperature of 70 °C for a minimum of 2 min, vegetative bacteria are efficiently killed. However, C. perfringens endospores are not completely inactivated even at temperatures as high as 100 °C. The heat resistance of the spores varies substantially depending on the strain and some can survive at 100 °C for up to 60 min. Furthermore, the heating can lead to efficient germination of spores followed by growth of the vegetative bacteria in insufficiently cooled food. This is why C. perfringens intoxications are often associated with catering events, where large amounts of food are prepared and stored for longer periods.

C. perfringens infections are most commonly associated with protein-rich meals like roasted meat, sauces, stews or soups prepared in advance in large amounts and cooled down too slowly. The prerequisite for an intoxication is the ingestion of a large amount of vegetative cells, which sporulate in the small intestine and subsequently produce toxins. After activation of the toxin(s) (e.g. by trypsin), they bind to intestinal epithelial cells and cause diarrhoea as well as abdominal cramps within 8 - 16 hours after consumption. The symptoms are self-limiting and disappear within one day.

Independent of the consumption of food, C. perfringens type F causes 3 - 15% of all human gastrointestinal infections, often associated with an antibiotic treatment.

Clostridium botulinum is a classical food intoxication pathogen; under anaerobic conditions in food it can produce a very strong neurotoxin which induces symptoms of paralysis in humans after consumption (food-borne botulism (PDF file,72.35 KB)) and has caused fatalities. Additionally, intoxication can occur due to the entry of bacteria or spores into wounds (wound botulism) and in infants after food intake with toxin production in the intestine (infant botulism).

C. botulinum-strains are divided into groups I - IV with only groups I and II being relevant with regard to food hygiene. The strains are further subdivided according to the production of neurotoxin types, which are assigned the letters A - G. Recently a new toxinotype, designated H, was described in the USA.
The proteolytic strains of group I multiply at temperatures between 10 and 42 °C, while the growth of non-proteolytic strains of group II are only inhibited at temperatures below 3.3 °C. Under anaerobic conditions and with suitable nutrition, the bacteria are able to form neurotoxins during growth - the classic example is insufficiently heated canned food. To prevent intoxication, canned food is normally heat treated during production (the so-called botulinum cook, 121 °C for 3 min). Spores belonging to group I are more heat-resistant than those belonging to group II. For prevention of spore germination and vegetative cell growth when food is unheated or insufficiently heated, strict compliance with the cooling chain procedures is a necessity. Alternatively, by heating directly prior to consumption it is possible to inactivate the heat sensitive neurotoxins. Aside from C. botulinum, some C. butyricum- and C. baratii-strains also have the ability to produce neurotoxins.

C. botulinum-strains belonging to group III mainly occur with botulism in domestic poultry or aquatic birds (type C) as well as in other (domestic) mammals (types C and D). Group IV has meanwhile been classified as a new species, C. argentinense, but has not yet been shown to be pathogenic either for humans or for animals.

Clostridium difficile (new taxonomic name: Clostridioides difficile) is an opportunistic pathogen that can trigger diarrhoea in animals and in humans having disturbances in the gut microbiota (frequently caused by antibiotic treatment during hospitalization or comorbidities of the intestine). Also for this species, the production of two exotoxins, toxin A and toxin B, is critical for disease onset. Another, so-called binary toxin (CDT), which occurs in some strains, is not able to induce disease on its own but can lead to more severe symptoms when occurring in addition to the classical toxins.

Prevalent multidrug resistant strains are difficult to treat and often lead to complicated disease progression and potentially death.

The transmission between humans and presumably also between humans and animals (zoonosis) occurs primarily via very robust, oxygen tolerant endospores which can survive in the environment for long periods of time. Given that C. difficile has also been detected in food and can withstand a heating of 70 °C for at least 2 min, which is normally recommended for the preparation of meals, its transmission via contaminated food is also a consideration. In the intestine, the spores germinate in the presence of primary bile salts and can, under certain conditions, multiply and produce toxins. Beyond the classic clinical symptoms of diarrhea, abdominal pain, and fever, severe complications like pseudomembranous colitis, toxic megacolon, an intestinal occlusion or perforation or a sepsis can occur in rare cases.

In addition to the clinically relevant Clostridium species there are also some classical spoilage agents, for example Clostridium estertheticum, which is able to grow even at low temperatures.