Microbiology Picture Quiz: Number 2: Answers

An Injecting Drug User presents to the ED with the lesion (as shown) on his arm. He also has a fever.


1) What disease is very important to exclude as a cause of his lesion?

Cutaneous anthrax would be an important diagnosis not to miss here. There are three main forms of anthrax, cutaneous, inhalational, and gastro-intestinal. Early treatment of cutaneous anthrax is associated with a high survival rate. Injecting Drug Users mainly suffer from either the cutaneous or inhalational forms of anthrax.

Cutaneous anthrax in its early stages may present as a papule,vesicle or ulcer (as opposed to the classical black eschar seen in the latter stages). They may be multiple, particularly in injecting drug users.


2) The clinician phones the microbiology laboratory asking what samples to take. What advice do you give?

  •  Superficial swab of the lesion for culture. (Further swab if PCR for B. anthracis is locally available)
  •  2 x punch biopsies, one in formalin for histology, the other for culture.
  •  2 set of blood cultures.
  •  Acute and convalescent serum samples for serological testing.
  •  Although not a “sample”, the clinician should be strongly encouraged to get a photograph of the lesion/s


3) How would you diagnose this condition in the laboratory?

B. anthracis is a Hazard Group 3 Pathogen and sample can be safely handled in containment level 3, in a class 1 safety cabinet.

Initially a Gram stain should be made after fixing a smear in 100% ethanol for one minute. The sample should also be cultured on routine media which should include straightforward blood agar.

A classical Gram stain is shows short chains of “boxcar” like Gram positive bacilli. Spore formation may or may not be present.

Colonies of B. anthracis (after 24 hrs incubation) are white, non-haemolytic with irregular edges and can be described as oval or comma shaped. They are also granular and “tacky”. They are catalase positive.

Any positive/suspicious cultures should be sent to a reference laboratory for confirmation.

As with all exotic diseases like this it is important to seek expert assistance as soon as possible, both from a laboratory and clinical perspective. Public Health Authorities should also be made aware of the case at an early stage.

The above answers are just a summary of the complete picture….. Here are two good references for more detailed information on the diagnosis of anthrax in all its forms.


Laboratory Investigation of Possible Anthrax in Heroin Users: HPA UK

CDC Guidelines: Diagnosis of Anthrax





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