You’re a physician trying to isolate bacterial colonies from the human gut in at
ID: 133177 • Letter: Y
Question
You’re a physician trying to isolate bacterial colonies from the human gut in attempt to diagnose a gastrointestinal infection. You streak your sample on a growth media containing glucose, amino acids, and salts that contain both sulfur and phosphorous with a pH of 7. You incubate the plates in aerobic conditions at 37 ?C for three days, at which point you can see clear bacterial colonies forming on the plate. Would you feel confident in stating that you had successfully cultured all the bacteria from your gut sample? Why or why not? (5 points)
Explanation / Answer
Bacteria that cause gastrointestinal infections are mainly Campylobacter, Salmonella, Shigella, Vibrio cholera, Esherechia (Enterohemorrhagic or Enterotoxigenic) etc
We cannot be confident that all the bacteria from the gut sample are isolated successfully because:
1. The most common bacterial infection is due to Campylobacter which is first isolated by growing in selective media and then incubation at 41.5 0 C for 44 hours under microaerobic conditions with slightly alkaline pHwhich is not provided and hence cannot be isolated by this physician and the pathogen likely doesnt survive in this sulfur and phosphorous reach medium.
2. Gram negative broth, Rapport broth and Selenite F broth are used as selective enrichment media for inhibiting the growth of normal flora and allows only pathogenic bacteria.
Among this, gram negative broth contains deoxycholate which is inhibitory for E.coli and helps in isolation of Shigella which generally utilizes 4-6 hour incubation period.
Selenite broth contains Sodium selenite, and useful for elimination of E.coli, Coliform bacillli. It helps in growth of Salmonella with 8-12 hour incubation.
Rapport broth contains magnesium chloride, malachite green which shows more selectivity than other media and used for Salmonella isolation. It is incubated for 18-24 hours
As seen from the above description none of the medium has high sulfur and phosphorous contents and incubation periods are different than used for isolation in the mentioned question. Hence, even though the physician in this case might be able to isolate some bacteria, some bacteria are not eliminated while some healthy flora are also seen in the media without selectivity.
3. Some bacteria like Helicobacteria pylori cant be isolated in this case, as the pH utilized in this medium and ingredients of media do not support its growth.
4. Isolation of Shigella like bacteria requires differential substrate like lactose and require Sodium thiosulfate, selective chemicals like bile salts, brilliant green dye to obtain the isolate.
5. Vibrio cholerae is grown on medium supplemented with thiosulfate, with pH 8.6 (to inhibit enterobacteriaceae) ox bile and cholate suppress enterococci with sucrose as substrate. Being sucrose fermenter vibrio converts bromothymol blue to yellow. Hence, vibrio is also not seen in the isolate.
6. Mac Conkey agar medium is used for isolatio of E.coli with lactose as substrate (but not glucose)
Thus, it can be fairly concluded that, the physician mentioned in this question will be able to isolate only few bacteria but not all of them from the gut sample as the medium and conditions used are more general and not selective.
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