Direct Detection Methods

Other than Gram stain of patient specimens, there are no specific procedures for the direct detection of Bacillus spp. in clinical material. The organisms are grampositive when stained from young cultures but become gram-variable or gram-negative with age (Figure 18-1).

Figure 18-1 Gram stain of Bacillus cereus. The arrow is pointed at a spore, which is clear inside the gram-positive vegetative cell.

Figure 18-2 Spore stain of Bacillus cereus. The arrows are pointed at green spores in a pink vegetative cell.

jW 1

Figure 18-2 Spore stain of Bacillus cereus. The arrows are pointed at green spores in a pink vegetative cell.

A feature of Bacillus spp. that is unique from all other clinically relevant aerobic organisms is the ability to produce spores in the presence of oxygen. Although spo :s are not readily evident on all smears containing Bacillus spp., their presence confirms the genus identification.

To induce sporulation, organisms can be grown on triple sugar iron agar (tsi), urea agar, or nutrient agar Containing 5 mg manganese sulfate per liter. On Gram-stained smears, spores appear clear because they do not retain the crystal violet or safranin. However, spores will stain with specific dyes such as malachite green, which is forced into the spore using heat; the Vegetative cell is then counterstained with safranin (Figure 18-2).

CULTIVATION Media of Choice

U1 Bacillus and related genera grow well on 5% sheep bit vd agar, chocolate agar, routine blood culture media, Irid Commonly used nutrient broths. They will not grow Oh MacConkey agar, and those that are susceptible to nalidixic acid will not grow on CNA (Columbia agar with nalidixic acid and colistin). PEA (phenylethyl alcohol agar) is useful for the isolation of Bacillus spp. from contaminated specimens.

TVvo special media are used for isolation and identification of B. anthracis. plet (polymyxin-Iysozyme-Idta. -thallous acetate) can be used for selection and isolation of this spedes from contaminated specimens. iC a means of identification, bicarbonate agar is used to induce B. anthracis capsule formation.

Incubation Conditions and Duration

Most spedes will produce detectable growth within 24 hours following incubation on media incubated at

35° C, in ambient air, or in 5% carbon dioxide (C02). Bicarbonate agar must be incubated in C02.

Colonial Appearance

Table 18-3 describes the colonial appearance and other distinguishing characteristics (e.g., hemolysis) of each spedes of Bacillus or related genera on blood agar. Colonies of B. anthracis growing on bicarbonate agar are large and mucoid.

Was this article helpful?

0 0
Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

Get My Free Ebook

Post a comment