5. Jessica and her lab partner have been given pure cultures of Staphylococcus a

Question

5. Jessica and her lab partner have been given pure cultures of Staphylococcus aureus and E. coli. Jessica's lab partner has already done the Nitrocefin Dry Slide ß-lactamase test and a Disk Diffusion (Kirby-Bauer) test on these two organisms and found that the S aureus was very susceptible to penicillin in a Disk Diffusion (Kirby-Bauer) test and did not produce B-lactamase. However Jessica's lab partner found that E. coli was much more resistant to penicillin but also did not produce ß-lactamase. Jessica's lab partner is confused by these results. Luckily Jessica remembers that when she Gram stained these two organisms she saw "purple" cocci with the S. aureus culture and "pink" rods from the E. coli culture. She also remembers that the Gram stain is based on differences in bacterial cell wall structure. Based on all the information she has Jessica correctly explains these results to her lab partner? What was Jessica's explanation for the results seen? 14 pts)

Explanation / Answer

Here Nitrocephin is introduced in culture of S.aureus and E.coli. S. aureus is gram positive bacteria and it produces purple color upon gram staining. E.coli is gram negative bacteria and produces pink color in gram staining. S.aureus is susceptible to penicillin . Similarly nitrocephin , a cephalosporin is beta lactum antibiotic . S.aureus is suceptible to nitrocephin and hence it doesnot produce beta lactamase. But E.coli is resistant to penicillin but doesnot show beta lactamase in the test. E.coli is resistant to nitrocephin and it should show presence of beta lactamase. this is because the discs are present close to each other , less than 20mm . Because of this the organism shows false susceptibility fo nitrocephin . A second test must be performed inorder to show that E.coli is resistant to cephalosporin The distance should be >20mm. thus , the above is the explanation for the results.

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