Gram Positive Bacilli: Spore Forming

Two genera produce endospores among medically important bacteria: Bacillus, aerobic or facultative anaerobic, and Clostridium and Clostridioides, obligate or aerotolerant anaerobic.

Endospores are highly resistant to heat, desiccation, and standard disinfection, requiring autoclaving for reliable destruction.

This group is dominated by toxin-mediated disease: several of the most potent toxins in microbiology are produced by organisms in this section.

Morphology. Large Gram positive rods, arranged in long chains, described as boxcar appearance, forming a prominent antiphagocytic capsule, unusual among Bacillus species, most of which are nonencapsulated, and central spores.

Important toxins. A tripartite toxin system: protective antigen binds host cell receptors and mediates entry of the other two components. Oedema factor is an adenylate cyclase, raising intracellular cAMP, disrupting fluid homeostasis, and causing local oedema. Lethal factor is a protease, disrupting intracellular signalling pathways and causing cell death. Protective antigen combines with oedema factor to form oedema toxin, and with lethal factor to form lethal toxin.

Transmission. Contact with spores from infected animals or contaminated animal products, wool, hides, meat. Inhalation of spores, historically associated with wool processing, woolsorter's disease, and with bioterrorism concerns given the organism's spore stability and lethality. Ingestion of contaminated meat.

Disease syndromes. Cutaneous anthrax is the most common natural form, a painless black eschar at the inoculation site, malignant pustule. Inhalational anthrax is rare but highly lethal, with initial nonspecific symptoms progressing to severe mediastinitis and haemorrhagic pleural effusion, associated with bioterrorism scenarios. Gastrointestinal anthrax is rare, following ingestion of contaminated meat.

Diagnosis. Gram stain and culture from the relevant specimen, skin lesion, blood, sputum. Characteristic large boxcar-chain morphology on microscopy. PCR and serology are available in specialised and reference laboratories given the public health significance of confirmed cases.

Key microbiological features. Nonhaemolytic on blood agar, distinguishing it from B. cereus, which is typically haemolytic. Nonmotile, distinguishing it from most other Bacillus species, which are motile.

Morphology. Gram positive rods, motile, typically beta-haemolytic on blood agar.

Important toxins. Heat-stable emetic toxin is preformed in contaminated food, causing rapid-onset vomiting, classically associated with reheated rice. Heat-labile diarrhoeal enterotoxin is produced after ingestion, in the small intestine, causing a later-onset diarrhoeal illness.

Transmission. Ingestion of contaminated food, particularly reheated or improperly refrigerated starchy foods, rice being the classic example, given the organism's spore survival through initial cooking and subsequent germination and toxin production if the food is left at room temperature.

Disease syndromes. Two distinct food poisoning syndromes reflect the two toxin types: emetic syndrome, onset one to six hours, vomiting-predominant, resembling S. aureus food poisoning in timing, and diarrhoeal syndrome, onset eight to sixteen hours, diarrhoea-predominant, resembling Clostridium perfringens food poisoning in timing. Also causes ocular infection, endophthalmitis, following penetrating eye trauma, and, rarely, invasive disease in immunocompromised patients.

Diagnosis. Clinical diagnosis in most cases of food poisoning, supported by isolation of the organism from the implicated food or from stool in outbreak investigation.

Key microbiological features. Motile and beta-haemolytic, both distinguishing it from B. anthracis.

Morphology. Gram positive rods, terminal spores producing a characteristic drumstick or tennis racquet appearance, obligate anaerobe.

Important toxins. Tetanospasmin is an A-B neurotoxin that travels retrogradely along peripheral motor neuron axons to the spinal cord, then blocks release of the inhibitory neurotransmitters glycine and GABA at inhibitory interneuron synapses, producing sustained, uncontrolled muscle contraction, spastic paralysis, rather than flaccid paralysis.

Transmission. Spore inoculation into a wound, particularly deep, devitalised, or contaminated wounds providing an anaerobic environment for germination. Neonatal tetanus results from umbilical stump contamination in inadequately hygienic delivery or cord care settings.

Disease syndromes. Generalised tetanus presents with trismus, lockjaw, as an early sign, progressing to generalised muscle rigidity and spasms, risus sardonicus, opisthotonus, and potentially fatal respiratory or autonomic involvement. Localised tetanus produces rigidity confined to the muscles near the wound, less common. Neonatal tetanus is a major cause of neonatal mortality in settings without adequate maternal immunisation and clean delivery practice.

Diagnosis. Primarily clinical, based on the characteristic pattern of rigidity and spasm in the appropriate epidemiological context, wound history, immunisation status. Culture is insensitive and rarely positive, and is not relied upon for diagnosis.

Key microbiological features. The toxoid form of tetanospasmin is the basis of the tetanus vaccine, a cornerstone of routine childhood and wound-management immunisation.

Morphology. Gram positive rods, subterminal spores, obligate anaerobe.

Important toxins. Botulinum toxin is an A-B neurotoxin and the most potent biological toxin known. It acts at the peripheral neuromuscular junction by cleaving SNARE proteins required for acetylcholine vesicle fusion and release, blocking acetylcholine release and producing flaccid paralysis. This is the direct opposite mechanism and clinical pattern to tetanospasmin.

Transmission. Foodborne botulism results from ingestion of preformed toxin in improperly canned or preserved food, classically home-canned low-acid vegetables, where anaerobic conditions and inadequate heat processing allow spore germination and toxin production.

Infant botulism results from ingestion of spores, not preformed toxin. The spores germinate and produce toxin in the infant gut, classically associated with honey ingestion in infants under twelve months, which is why honey should not be given to infants under one year. Wound botulism results from spore contamination of a wound, with in situ toxin production, associated particularly with injecting drug use.

Disease syndromes. Descending, symmetric flaccid paralysis, typically beginning with cranial nerve involvement, diplopia, ptosis, dysarthria, dysphagia, before progressing caudally to limb weakness and, in severe cases, respiratory failure. Autonomic symptoms, dry mouth, constipation, also occur. Consciousness and sensation remain intact throughout, a distinguishing clinical feature.

Diagnosis. Detection of toxin in serum, stool, or the implicated food, mouse bioassay historically, increasingly replaced by other assays. Clinical diagnosis is often required to guide urgent antitoxin treatment before laboratory confirmation is available, given the time-critical nature of the disease.

Key microbiological features. The same toxin, in carefully controlled minute doses, is used therapeutically and cosmetically, botulinum toxin injections, for conditions involving excessive or unwanted muscle activity.

Morphology. Gram positive rods, spores rarely seen on Gram stain of clinical specimens, obligate anaerobe, though relatively aerotolerant compared to some other clostridia, nonmotile, typically displays a double zone of haemolysis on blood agar.

Important toxins. Alpha toxin is a phospholipase, lecithinase, that degrades host cell membranes, the principal toxin responsible for the extensive tissue necrosis of gas gangrene. A separate heat-labile enterotoxin is produced in the intestine after ingestion of large numbers of vegetative cells, responsible for food poisoning.

Transmission. Ubiquitous in soil and as part of normal gastrointestinal flora. Wound contamination, traumatic or surgical, particularly with devitalised tissue providing an anaerobic environment, for invasive disease. Ingestion of contaminated, inadequately reheated meat or gravy dishes for food poisoning.

Disease syndromes. Gas gangrene, clostridial myonecrosis, is a rapidly progressive, life-threatening muscle infection with gas production in tissue, crepitus on examination, severe pain, and systemic toxicity, a true surgical emergency requiring urgent debridement alongside antibiotics. Food poisoning is a relatively mild, self-limited diarrhoeal illness with onset eight to sixteen hours after ingestion.

Diagnosis. Gas gangrene is largely a clinical and surgical diagnosis, crepitus, tissue appearance at debridement, characteristic imaging showing gas in tissue planes, supported by Gram stain and culture. Food poisoning is diagnosed clinically and epidemiologically, supported by organism or toxin detection in stool or the implicated food where indicated.

Key microbiological features. Double-zone haemolysis on blood agar, a wide zone of complete beta-haemolysis from theta toxin, surrounding a narrower zone of incomplete haemolysis from alpha toxin, is a classic, distinguishing laboratory feature.

Morphology. Gram positive rods, spore-forming, obligate anaerobe.

Important toxins. Toxin A, enterotoxin, and toxin B, cytotoxin, are both glucosylating toxins that inactivate Rho-family GTPases within host colonic epithelial cells, disrupting the actin cytoskeleton, causing cell rounding, loss of tight junction integrity, and an intense inflammatory response. Hypervirulent strains, notably ribotype 027, produce increased toxin quantities and are associated with more severe disease and outbreaks.

Transmission. Faecal-oral, via spores that survive gastric acid, alcohol-based hand sanitiser, and standard environmental disinfection. Healthcare-associated transmission via contaminated surfaces, equipment, and the hands of healthcare workers is the dominant route in most symptomatic disease.

Disease syndromes. Classic antibiotic-associated dysbiosis pathogen. Disruption of normal colonic flora by antibiotic therapy allows ingested or already-present C. difficile spores to germinate and overgrow. This produces toxin-mediated colitis ranging from mild watery diarrhoea to severe or fulminant pseudomembranous colitis. Features include adherent inflammatory pseudomembranes visible on colonoscopy, toxic megacolon, and, in severe cases, bowel perforation and death.

Diagnosis. Stool testing for toxin, enzyme immunoassay, and/or toxin gene detection, PCR, often combined with a glutamate dehydrogenase, GDH, screening test in a two-step or multi-step algorithm. GDH is highly sensitive but not specific for toxin-producing strains, while toxin and PCR testing confirms clinically significant, toxigenic infection.

Testing should only be performed on unformed or liquid stool, since detecting the organism or its genes in a formed stool sample more often reflects asymptomatic colonisation than active disease.

Key microbiological features. Because alcohol-based hand sanitiser does not reliably kill C. difficile spores, soap-and-water hand hygiene is specifically required when caring for a patient with known or suspected C. difficile infection, a direct exception to standard alcohol-gel-based infection control practice.