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A Guide To The Recognition Of Bacillus Cereus Biology Essay

Bacillus cereus can be an endospore-forming, gram-positive bacterium that commonly resides in the garden soil. Due to its location, Bacillus cereus is usually found on a variety of foods that come into close contact with contaminated garden soil, and can cause two different kinds of food borne diseases: emetic and diarrheal. The type of illness that Bacillus cereus can infect a person with depends mainly on the sort of contaminated food that is ingested (Cowan and Talaro, 2009). Strains that produce the emetic form of the disease grow mainly in starchy foods like grain and potatoes; especially after they have been cooked properly and held warm for a long period of time. Following the ingestion of such polluted foods, Bacillus cereus may incubate inside the afflicted organism for up to six time before showing any observeable symptoms that the emetic form displays like nausea, throwing up and stomach cramps (Drobniewski, 1993). Strains that produce the diarrheal form of the condition grow in a variety of foods ranging from vegetables to even meat products. "Patients experience profuse diarrhea with belly pain and crampswhich get started around eight to sixteen time after ingestion of the polluted food" (Drobniewski, 1993). Recovery is usually immediate for both types of diseases; around twelve to a day after the symptoms have emerged. "Diagnosis of the emetic form of the condition is achieved by locating the bacterium in the implicated food source. Microscopic examination of stool examples is regular to analyze the diarrheal form of the disease. In both circumstances, the only prevention is the correct handling of food" (Cowan and Talaro, 2009).

The Bacillus species are gram-positive, "rod-shaped, and frequently organized in pairs or chains, with round or square ends. Endospores are oval or sometimes cylindrical" (Harley, 2008). For the case of Bacillus cereus, this rod-shaped bacterium is commonly organized in chains with rectangular ends and contains terminal endospores. On a normal TSA plate, the colonies of Bacillus cereus appear to have a cream "dull or frosted wine glass appearance and frequently an undulate margin from which comprehensive outgrowths do not develop" (Sneath, 1986). "The genus Bacillus is either aerobic or facultatively anaerobic, most users of the group are also motileand with the exceptions of Bacillus anthracis and Bacillus cereus, are nonpathogenic" (Harley, 2008).

When trying to identify Bacillus cereus, there are several checks and results that will help appropriately identify this bacterium (Stand 1). The first method that should be done is to streak the bacterias for isolation over a TSA plate. To become in a position to view the right morphology of any bacteria, one must have a brand new 24 hour culture formulated with individual colonies for proper Gram staining. Gram staining is a method used to recognize the cell morphology of a specific bacterium and differentiates bacterias as gram-positive and gram-negative cells. As stated preceding, Bacillus cereus is a gram-positive rod-shaped bacterium that is established in chains. Once the cell morphology of the bacterium is set, an endospore stain would be another test used in order to determine whether or not it is an endospore or non-endospore forming bacterium. Bacterias in the genera Bacillus and Clostridium can produce endospores, that happen to be structures "capable of surviving for very long periods within an unfavorable environment and then supplying rise to a fresh bacterial cell" (Harley, 2008). As a result, by being in a position to classify the bacterium as endospore-forming, many sets of bacteria can be eliminated by narrowing it down it the two different genera previously stated. Since endospores are being used as a kind of survival when nutrients are being depleted, a four to six day culture may be easier to use so that the endospores have had some time to totally develop. Air requirements would be the next test because bacterias in the Bacillus genus can either be obligate aerobes or facultative anaerobes, and bacterias in the Clostridium genus are obligate anaerobes. Bacillus cereus can either develop only at the top of thioglycollate broth, and therefore it is an obligate aerobe, or it could display expansion throughout the broth with a larger amount on the surface, meaning that this is a facultative anaerobe. Motility is also an important attribute used to recognize microorganisms and since not all Bacillus bacteria have this trait, the range of possible bacterias can be narrowed again with these results. Bacillus cereus is motile, since when this bacterium is inoculated in to the motility press, the growth of the bacterium is not restricted within the stab type of the inoculation but can be seen throughout the mass media. After deciding that the bacterium is positive for motility, fermentation reactions of both blood sugar and lactose should be achieved to establish whether or not the bacterium has the capacity to ferment either of these glucose, as well as produce gas through the process. True Bacillus cereus can ferment blood sugar but it cannot ferment lactose; none of them of the fermentation reactions produce gas as well.

Several problems can be encountered when trying to appropriately identify Bacillus cereus because of similar characteristics shared with Bacillus anthracis; both bacteria are gram-positive endospore-forming rods that are set up in chains. The variation between Bacillus cereus and Bacillus anthracis and their separation from nonpathogenic varieties are the most crucial clinically since both are pathogenic. "Generally, Bacillus cereus is motile, -hemolytic on bloodstream agar, and penicillin resistant, whereas B. anthracis is not. Bacillus cereus, unlike Bacillus anthracis, produces -lactamases, and so it is protected to -lactam antimicrobial realtors, like Penicillin" (Drobniewski, 1993). One method that is employed to test for Penicillin awareness is the Kirby-Bauer method; Penicillin-filled paper disks are positioned on a completely inoculated Mueller-Hinton agar bowl of the bacteria to see areas of inhibition (Harley, 2008). Bacillus anthracis will create a distinguishable zone of inhibition due to its awareness to Penicillin, whereas Bacillus cereus will either create a minor zone of inhibition or none of them at all because of its level of resistance to Penicillin. The stand you can use to identify a bacterium's sensitivity or level of resistance to Penicillin, predicated on the zones of inhibition, are available on webpage 265 of the Microbiology Laboratory Manual by John P. Harley.

Other than the tests previously reviewed for the recognition of Bacillus cereus, there are several assessments that can come with these to definitively identify the bacterium as Bacillus cereus. One test is a hemolysis test on blood vessels agar. The blood agar dish is a differential press that distinguishes between hemolytic and non-hemolytic bacteria. A hemolytic bacterium produces clear areas about the colonies scheduled to red blood vessels cell damage; ˜-hemolysis exhibits a partial clearing about the bacterial colonies, -hemolysis exhibits a total clearing across the bacterial colonies and -hemolysis displays no hemolysis, therefore, no clearing surrounding the colonies. "Typical isolates of Bacillus cereus on blood vessels agar form large, smooth, granular, ground wine glass, -hemolytic colonies" that screen a slight renewable tinged color (Drobniewski, 1993). Another test that can be done to appropriately identify the bacterium as Bacillus cereus is its appearance on PEMBA (Polymixin pyruvate egg yolk mannitol bromothymol blue agar) multimedia. On the agar, Bacillus cereus varieties crenate or fimbriate to slightly rhizoid colonies. They have particular turquoise to peacock blue color because of the absence of mannitol fermentation and are usually ornamented by an egg yolk precipitate of similar color credited to lecithinase development (Sneath, 1986). Almost every other people of the Bacillus genera are mannitol positive, show up as yellowish colonies and do not produce lecithinase.

Due to its pathogenicity, proper recognition of Bacillus cereus in contaminated foods is important to avoid food poisoning. By performing these exams and traffic monitoring the results, a tension of Bacillus cereus can be effectively identified.

Table 1. Biochemical testing and the expected results used to effectively identify Bacillus cereus.

Biochemical Test


Gram Stain









A and NG


NA and NG



A=Acid production, NA= No acid solution production, G= Gas creation, NG= No gas creation, FA= Facultative Anaerobe, (+)= Positive for biochemical test, (-)= Negative for biochemical test

Literature Cited

Cowan M, Talaro K. 2009. Microbiology A Systems Methodology. New York: McGraw-Hill. 869 p.

Drobniewski F. 1993. Bacillus cereus and Related Species. Clinical Microbiology Reviews. (6)4: 324-338.

Harley J. 2008. Lab Exercises in Microbiology. New York: McGraw-Hill. 486 p.

Sneath P. 1986. Endospore-forming Gram-positive Rods and Cocci. In: Sneath, PH, editor. Bergey's Manual of Systematic Bacteriology, vol. 2. Baltimore: Williams and Wilkins.

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