OVERVIEW: What every clinician needs to know

Pathogen name and classification

Bacillus spp., Family Bacillaceae, Phylum Firmicutes.

What is the best treatment?

  • There is no treatment required for the food poisoning syndromes. In general, many classes of antibiotics are useful, but this is somewhat species-dependent. Most beta-lactams are not active in vitro against B. cereus although carbapenems look good. Fluoroquinolones look to be active against B. sphaericus.

  • There are potential issues of antibiotic resistance, but there is no specific set of concerns that will not be apparent by determining the species and performing routine antimicrobial susceptibility testing.

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    In its spore form, Bacillus demonstrates resistance to almost all antimicrobials as well as to extremes of heat and dryness. In the vegetative form, there are several mechanisms of resistance. B. cereus produces a beta-lactamase that confers resistance to most penicillins and cephalosporins but retains susceptibility to carbapenems. There are plasmid-based mechanisms of resistance for tetracyclines. Resistance to vancomycin is not reported and resistance to fluoroquinolones is still rare.

    There are Clinical and Laboratory Standards Institute (CLSI) guidelines for susceptibility testing.

    If treatment is indicated, the agents most likely to work include fluoroquinolones and carbapenems. It may also be important to remove foreign bodies and/or to drain pus because Bacillus spp. can thrive within biofilm and may be difficult to eradicate without removal of the spores and vegetatively dormant members of the biofilm community.

How do patients contract this infection, and how do I prevent spread to other patients?

  • Epidemiology-

    No seasonal variance.

    Bacillus spp. are found in most natural environments. Because they form spores, they can tolerate extremes of temperature, humidity, etc. They also form a small part of normal human flora. In both the food poisoning and pyogenic settings, they afflict people because of their ubiquitous presence in the environment.

    Infection is rare worldwide. As a cause of food poisoning, the various Bacillus-related intoxications are less common that Salmonella, Shigella, E. coli, Campylobacter, Clostridium perfringens and Staphylococcus aureus. Pyogenic infections are rare and may have a slight predilection for tropical areas.

    The incidence seems stable over time.

  • Infection control issues-

    Normal protective measures suffice since there is not a particular risk of disease from casual exposure. For avoidance of food poisoning, the usual food preparation hygiene measures will be perfectly adequate. There is one uncommon manifestation of Bacillus cereus food poisoning that may be preventable: not using previously cooked rice maintained at room temperature for reheated dishes such as fried rice. One of the B. cereus toxins can be elaborated in the starchy environment of rice and is not destroyed by heat.

    There are no vaccines available.

    There is no specific prophylaxis needed. In terms of the prevention of pyogenic infections, normal sterile technique and the usual forms of prophylaxis are adequate.

What host factors protect against this infection?

  • There is no known specific host defect that predisposes to this rare infection. In the limited setting of persistent Bacillus spp. bacteremia, biofilm on a vascular catheter is a likely scenario and removal of the catheter is very helpful in resolving the infection.

  • There is no special risk group.

  • There is nothing pathognomonic about either the food poisoning or pyogenic syndromes caused by Bacillus spp. Tissue stains may show typical boxcar shaped organisms and/or spores, but these are not universally found.

What common complications are associated with infection with this pathogen?

  • Food poisoning is a self-limited short duration disease, normally involving some combination of nausea, vomiting and diarrhea. Pyogenic infections can follow eye injury or surgery, and have local manifestations in the globe. Disease elsewhere in the body affects the involved organ in much the same way that other bacterial infections do.

  • There is a propensity of these organisms to cause eye infections. The route of entry is usually surgery or ocular trauma.

How should I identify the organism?

  • This bacterium can live vegetatively in many environmental niches and forms spores readily.

  • Normally processed bacterial cultures are effective in detecting all Bacillus spp. The usual concern is whether the specimen represents true infection or simple contamination of the culture specimen. Finding multiple positive cultures, heavy growth and a compatible clinical syndrome is very useful.

  • Bacillus spp. are easily identified as Gram-positive rods by Gram stain. They tend to be large organisms and have a dark blue and often boxcar shaped appearance. Seeing spores will confirm that the organism is either Bacillus or Clostridium. Since Clostridium is anaerobic, the laboratory distinction can be readily made by culture if the Gram stain is inconclusive.

  • Routine bacterial cultures on blood agar or chocolate agar are sufficient. The organism will also grow in broth and in blood culture media.

    Blood agar.

    Colonies are often large and grayish white. They can have irregular margins.

    Bacillus spp. have a variety of biochemical reactions and some species such as B. sphaericus and B. badius are fairly biochemically inert. In terms of distinguishing B. cereus from the genetically closely-related but clinically distinct B. anthracis, motility and hemolysis are very important: in both cases, B. cereus will test positive and B. anthracis will test as being non-motile and non-hemolytic. Normal commercial laboratory protocols can often identify Bacillus to the species level but rare species may require a reference laboratory for species determination.

    Bacillus spp. typically grow quickly, and an overnight incubation is adequate to see a normal colony.

    Cultures are quite sensitive. In fact, Bacillus spp. grow so well that sometimes minor contamination can be picked up.

  • Polymerase chain reaction (PCR) and other molecular techniques are not routinely used for Bacillus spp.

  • In rare instances, phage testing, chromatography, mass spectroscopy and testing for flagellar antigens can be useful.

How does this organism cause disease?

  • Unlike B. anthracis that contains significant toxins that account for its pathogenic potential, most invasive infections caused by Bacillus spp. are felt to be largely related to the presence of the organism and the tissue response. There is no person-to-person transmission. Tissue destruction seems to be greatest when there is foreign material or non-viable tissue for the organism to persist in.

  • Toxins are responsible for the food poisoning manifestations. The diarrheal form of B. cereus food poisoning appears to be caused by two or more proteins. They are quickly cleared from the gastrointestinal (GI) tract and the clinical manifestations disappear fast — usually within 24 hours. The emetic form of B. cereus food poisoning is caused by a small toxin molecule that is heat stable. There are less common food poisoning associated with B. licheniformis, B. subtilis and B. pumilus, and their mechanism is less well worked out.

Management and treatment

Treatment of pyogenic infection is usually accomplished by removal of foreign material (if present and possible to remove) and addition of antibiotics. There are no structured studies comparing various regimens so the choice of drug is based either on susceptibility test of actual isolates or general empiric rules if there is no opportunity to perform in vitro susceptibility testing. Antibiotic susceptibility is variable and sometimes predicted by species. B. cereus tends to be resistant to most beta-lactams except carbapenems. Vancomycin is usually active and fluoroquinolones are also recommended. Clindamycin has good in vitro and in vivo activity, although some strains are resistant.

Treatment of food poisoning syndromes is symptomatic only. There is little evidence that viable bacteria are present in patients with either of the two major food poisoning types.

For endophthalmitis, aggressive therapy including antibiotics and vitrectomy can improve outcome. Keratitis from minor trauma or contact lens use is managed with less aggressive therapy including topical antibiotics such as fluoroquinolones.

WHAT’S THE EVIDENCE for specific management and treatment recommendations?