Gram-positive rods can mainly be classified based on several factors:
- Relationship to oxygen- Bacteria may be obligate aerobes (need oxygen to survive), obligate anaerobes (need an absence of oxygen in order to survive), or facultative anaerobes (can survive with or without oxygen).
- Spore formation- Some bacteria form spores that help them survive in harsh environmental conditions for many years. Such spores may also be infective.
- Morphology- Not all rod-shaped bacteria are created equally. Some rods look pretty normal, whereas others might be curved, branched, and so on.
There are two main endospore-forming bacteria: Bacillus and Clostridium. Bacillus is aerobic, whereas Clostridium is anaerobic.
In terms of regular-shaped non-endospore forming bacteria, Listeria and Erysipelothrix are aerobic and Lactobacillus is facultatively anaerobic.
In terms of irregular-shaped non-endospore forming bacteria, Corynebacterium is aerobic whereas Propionibacterium is anaerobic.
In terms of branching non-endospore forming bacteria, Nocardia, Actinomadura, and Streptomyces are aerobic, whereas Actinomyces is facultatively anaerobic.
Name the clinically important species within these genera and describe diseases they typically produce
Bacillus
There are several different groups of Bacillus. The group that we will be focusing on is the Bacillus cereus group.
The main member of the Bacillus cereus group is Bacillus cereus. It can cause food poisoning via enterotoxins. You are most likely to acquire Bacillus cereus from reheated rice dishes that have been contaminated.
Bacillus thuringiensis is another member of the Bacillus cereus group. B. thuringiensis is interesting in that it does not affect humans, but is very useful to us. You see, B. thuringiensis has cry genes that produce toxic crystals within spores. These toxic crystals can be used to kill insects. Thus B. thuringiensis can be used as an insecticide.
Bacillus anthracis, which causes anthrax, is another important member of the Bacillus cereus group. It lives in the soil and affects animals, particularly herbivores. It can be found worldwide, but in Australia cases are rather sporadic and mainly occur in the "Anthrax Belt" in central New South Wales. Anthrax can be acquired via the cutaneous, inhalational, or ingestional (gastrointestinal) routes.
Cutaneous anthrax is the most common form of anthrax and usually forms lesions called eschars. It has a mortality rate of 10-40% if left untreated. Inhalational anthrax survives in macrophages and is carried to lymphatics in the mediastinum, resulting in widening of the mediastinum. It is almost 100% fatal if untreated. Finally, gastrointestinal anthrax is the least common form of anthrax. It can cause inflammation, swelling, and haemorrhage of the gastrointestinal tract, with a mortality of around 50%.
Clostridium
Clostridium, like Bacillus, produces spores. Clostridium species generally produce toxins.
One important type of Clostridium is C. perfringens, which can be broken down into five types depending on which toxins are produced. The most important types in human disease are A and C. Type A C. perfringens can cause gas gangrene, in which spores rapidly invade and cause liquefactive necrosis and gas production in the tissue. Type A C. perfringens can also cause food poisoning via the ingestion of contaminated meat. Type C C. perfringens causes a condition called necrotising bowel disease, or enteritis necroticans if you want to be fancy about it. Type C is relatively uncommon but may occur when contaminated food is eaten in conjunction with food that is rich in trypsin inhibitors, as trypsin inhibitors prevent the breakdown of the bacterial toxins.
C. tetani is a species of Clostridium that is found in soil. It can release tetanospasmin, which is a powerful neurotoxin that blocks inhibitory impulses. Since inhibitory impulses are blocked, patients have increased muscle tone and spasms, resulting in trismus (lockjaw), risus sardonicus ("sardonic grin"- teeth clenched and lips pulled back), opisthotonus (arched back), and so on. Tetanus is thankfully rare due to vaccination. If contracted, it has a mortality rate of 40% or more.
C. botulinum releases botulinum toxin, causing botulism. Botulinum toxin prevents the release of ACh at the neuromuscular junction, resulting in paralysis. C. botulinum may be found in aged or preserved foods.
C. difficile is a common nosocomial (hospital-derived) infection that may be spread between patients via the hands of staff. Strains of C. difficile that produce enterotoxins can cause "antibiotic-associated diarrhoea," which is severe, intractable diarrhoea during or after antibiotic treatment. This occurs because there isn't enough commensal bacteria to compete for resources. C. difficile may also cause pseudomembranous colitis, which is a severe inflammatory condition of the colon.
Lactobacillus
Lactobacillus is part of the commensal flora of the vagina and gastrointestinal tract and is rarely pathogenic. It can ferment carbohydrates to lactic acid, maintaining vaginal pH between 3.8-4.5. A lack of Lactobacillus may result in an increase in pH.
Listeria
Listeria is a cause of food poisoning. Listeriosis is often asymptomatic, except in people who are immunocompromised in some way. However, when listeriosis hits, it hits pretty hard, causing septicaemia (bacteria multiplying in the bloodstream) and/or meningitis. If a pregnant woman is infected, her fetus may be infected too, resulting in miscarriage or stillbirth. If the fetus manages to survive to term, the baby may be severely ill. Since Listeria is often carried by certain high-risk foods (e.g. soft cheeses), pregnant women are often advised to avoid these foods.
Erysipelothrix
Erysipelothrix rhusiopathiae causes erysipeloid, which consists of painful, raised areas of inflammation of the skin. It is found in meat, poultry, fish, and so on, and enters the skin via abrasions.
Corynebacterium
Corynebacteria are small, irregular, Gram-positive rods that are often commensals in humans and animals. Corynebacteria are often found contaminating blood cultures, but sometimes they might actually be the pathogen causing the infection. The most common Corynebacterium is Corynebacterium diphtheriae, which causes diphtheria. In diphtheria, exotoxin-producing strains of C. diphtheriae infect the nose and throat, causing formation of a pseudomembrane in the nasopharynx. Fortunately, diphtheria is quite rare in the developed world due to vaccination.
Propionibacterium
Propionibacterium is mostly commensal and non-pathogenic, but can cause annoying stuff like acne (in the case of P. acnes). It can also infect foreign devices such as prosthetic heart valves and joints.
Actinomycetes
Actinomycetes are a diverse group of bacteria that can be found in soil and rotting vegetation. There are many different genera of actinomycetes, too many to list here, but they are mostly branching/filamentous Gram-positive rods.
One condition caused by actinomycetes is actinomycotic mycetoma, which is usually caused by the genera Nocardia, Actinomadura, and Streptomyces. Actinomycetes enter the skin and take years to form a swelling. (Plenty of gross pictures on Google if you want a better idea of what this looks like.)
Nocardia can also cause other illnesses. In immunocompromised hosts, it can cause a range of horrible problems, such as respiratory infections and abscesses in various organs. Even immunocompetent hosts aren't safe: Nocardia can implant in the skin and cause lesions in a "sporotrichoid" appearance (so-called because sporotrichosis, caused by a fungus, looks similar).
Finally, actinomycosis, characterised by hard swellings, is caused by the genus Actinomyces.
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