Bacillus anthracis

Associated infections

• Cutaneous anthrax
• Inhalational anthrax
• Gastrointestinal and oropharyngeal anthrax
• Injectional anthrax
• Anthrax meningitis
• Anthrax bacteraemia and systemic anthrax

Transmission route

Human anthrax follows direct or indirect contact with infected animals and their products. Cutaneous disease, which accounts for the large majority of naturally occurring cases, arises from spore entry through broken skin during handling of hides, wool, hair, bone meal or carcasses, and produces the characteristic painless ulcer with a black eschar and marked surrounding oedema. Inhalational disease follows aerosolisation of spores in occupational settings such as wool and hide processing, or from deliberate release, and presents with a biphasic course that progresses to haemorrhagic mediastinitis and shock. Gastrointestinal and oropharyngeal disease follows consumption of undercooked meat from an infected animal. Injectional anthrax has been described among people who inject heroin contaminated with spores, presenting as severe soft-tissue infection without the classical eschar.

Person-to-person transmission does not occur other than rarely by direct contact with cutaneous lesion exudate, and standard precautions are sufficient for the care of most patients. There is no established transmission route through reticulated or treated water supplies. Anthrax spores may be present in surface water and soil runoff in enzootic areas, but they do not multiply in water, are removed by conventional filtration and are inactivated by adequate treatment, and no waterborne outbreak of human anthrax through a potable supply is on record.

There is likewise no reported association between B. anthracis and endoscope reprocessing, automated endoscope reprocessor circuits, final rinse water, CSD water systems, dental unit waterlines or premise-plumbing biofilm. The organism has no ecological reason to be present in a treated water system in a healthcare facility, and its recovery from such a system would not be explicable by any recognised route of environmental ingress in the Australian setting. This absence of a plausible pathway is itself an important piece of interpretive evidence when a presumptive result is being assessed.

Relevance in endoscopy and reprocessing

There is no documented case of anthrax transmitted through flexible endoscope reprocessing, CSD instrument processing or dental water, and no pseudo-outbreak attributable to B. anthracis in any of those settings. The organism is not a target of reprocessing water surveillance, is not present in the mucosal flora that endoscopes contact, and has no described role in the biofilm communities that colonise purified water distribution systems and reprocessor circuits. On the evidence available, B. anthracis has no operational relevance to endoscope reprocessing as an infection risk.

What does have operational relevance is the wider genus. Aerobic spore-forming bacilli are frequently recovered from reprocessing water and from processed instruments, and they matter for two reasons. First, they contribute to total viable count and can therefore drive an exceedance without any Gram-negative or mycobacterial organism being present. Second, the bacterial spore is the most resistant form encountered in routine reprocessing, and high-level disinfection is by definition not a sporicidal process at the contact conditions used in automated endoscope reprocessor cycles. A spore-forming organism recovered from a processed endoscope channel or from final rinse water is not evidence of disinfectant failure in the ordinary sense, because the disinfectant was never expected to eliminate it; it is evidence about water quality, terminal filtration and cleaning efficacy.

The implications for practice follow from that. Spores are removed physically rather than chemically, so the controls that matter are the integrity and change interval of terminal filters, the condition of the distribution loop and its sanitisation regime, avoidance of dead legs and stagnation where sediment and spores accumulate, and thorough manual cleaning of instruments. Drying and storage are relevant because spores tolerate desiccation indefinitely, so a spore deposited on a channel surface will still be present at the end of a storage interval. In CSD the position differs, since steam sterilisation is sporicidal and Bacillus species are in fact used as the biological indicator organisms by which sterilisers are validated, which is a useful reminder that the presence of an aerobic spore-former in a flexible endoscopy setting reflects the intrinsic limits of high-level disinfection rather than a procedural lapse.

Interpreting a detection

This section carries the page. In a reprocessing facility, the practical question is almost never whether anthrax is present; it is how to handle a presumptive Bacillus result correctly so that a routine environmental finding is neither escalated into a false incident nor, in the very rare converse case, dismissed when it should not be. A presumptive Bacillus isolate from final rinse or supply water is a routine environmental finding and should be assessed on ordinary terms, principally as a contributor to total viable count and as an indicator of the general condition of the water system, of terminal filtration, or of sampling technique. It is not an anthrax signal, and the isolation of a Bacillus species should not by itself trigger notification, unit closure or public communication.

Definitive exclusion in a sentinel laboratory rests on a defined rule-out algorithm. The features that exclude B. anthracis are the presence of beta-haemolysis on sheep or horse blood agar, the presence of motility, and inconsistent Gram or colonial morphology; B. anthracis is characteristically non-haemolytic and non-motile with flat, ground-glass, tenacious colonies, whereas most environmental Bacillus species are haemolytic, motile or both. An isolate that is haemolytic or motile is excluded and requires no further action beyond ordinary water-quality assessment. An isolate that cannot be excluded on those grounds must be referred to a reference laboratory for gamma phage lysis testing, capsule demonstration and molecular confirmation using assays that target the chromosome together with both the pXO1 and pXO2 plasmids. A single assay is not sufficient, because closely related members of the B. cereus group can carry anthrax-associated genes and because plasmid-cured or plasmid-bearing atypical strains exist. Laboratories should be alerted before referral, and manipulation of a suspect isolate should stop at the point the rule-out fails.

If a genuine identification of B. anthracis is confirmed by a reference laboratory, the matter ceases to be a water-quality investigation. It becomes a public health and biosecurity incident requiring immediate notification of the jurisdictional public health unit and law enforcement, and subsequent actions, including any sampling, decontamination, staff assessment and communication, proceed under their direction rather than through the facility's own remediation process. The facility's obligations at that point are to preserve the isolate and the original sample, to secure and document the sampling location and chain of custody without further disturbance, to stop routine remediation activity that would destroy evidence such as flushing or disinfecting the affected line, to identify staff who handled the sample so that exposure assessment can be performed, and to make no attempt at independent environmental sampling. Escalation for a routine Bacillus isolate, by contrast, follows normal water-quality logic: repeat sampling, review of filtration and sanitisation, and trending against the facility's action levels.

Antimicrobial resistance

Naturally occurring isolates of B. anthracis are generally susceptible to a broad range of agents including ciprofloxacin, doxycycline, penicillin, amoxicillin, clindamycin, rifampicin, vancomycin and linezolid, and fresh isolates from clinical anthrax are almost always penicillin susceptible on testing. Penicillin is nonetheless not used empirically, because the species carries chromosomal beta-lactamase genes that can be expressed, because inducible resistance has been demonstrated, and because engineered or unusual resistance cannot be excluded at presentation, particularly where a deliberate release is suspected. The organism is intrinsically resistant to extended-spectrum cephalosporins and to trimethoprim-sulfamethoxazole, which removes agents that might otherwise be reached for empirically.

Ciprofloxacin and doxycycline are therefore the standard choices for post-exposure prophylaxis and form the backbone of treatment, with susceptibility testing and de-escalation once results are available. Prophylaxis is prolonged because spores may persist in the respiratory tract and germinate late, and vaccination is used alongside antimicrobials in some exposure scenarios. Systemic disease is managed with combination therapy that includes a bactericidal agent and a protein synthesis inhibitor such as clindamycin or linezolid to suppress toxin production, with regimens adjusted for central nervous system penetration where meningitis is present, since anthrax meningitis carries very high mortality. Antitoxin products directed at protective antigen are available as adjuncts to antimicrobial therapy.

For a reprocessing facility, the relevant resistance property is not antimicrobial at all but the durability of the spore, which is shared with the environmental Bacillus species actually encountered in water sampling. Spores resist desiccation, ultraviolet exposure at ordinary doses, and the aldehyde and oxidising disinfectants used for high-level disinfection at their routine contact conditions, and they persist indefinitely on dry surfaces. Control in a water system is therefore physical rather than chemical: adequate filtration, absence of stagnation and sediment accumulation, and a sanitisation regime that reaches the whole distribution loop. Chemical escalation, in the form of higher disinfectant concentration or longer contact time in an automated endoscope reprocessor cycle, is not an effective answer to a spore-forming isolate and should not be the first response to one.