Question

(Need to explain B. cereus resistance to penicillin and ampicillin. This was most likely because B. cereus has been shown to possess β-lactamase (ref).)

Answer 1

Both ampicillin and penicillin are β-lactam antibiotics. They are a group of antibiotics which contain a β-lactam ring in their molecular structure. They kill bacteria by inhibiting the cell wall synthesis. They act by inhibiting the synthesis of the peptidoglycan layer of bacterial cell walls. The peptidoglycan layer is important for cell wall structural integrity, especially in Gram-positive organisms, being the outermost and primary component of the wall. The final transpeptidation step in the synthesis of the peptidoglycan is facilitated by DD-transpeptidases which are penicillin binding proteins. The effectiveness of these antibiotics relies on their ability to reach the Penicillin Binding Proteins (PBP) intact and their ability to bind to the PBP.

B.cereus, which is a gram positive bacteria has developed resistance against these antibiotics, because they produce the enzyme β-lactamase. The mechanism is by Enzymatic hydrolysis of the β-lactam ring.If the bacterium produces the enzyme β-lactamase or the enzyme penicillinase, the enzyme will hydrolyse the β-lactam ring of the antibiotic, rendering the antibiotic ineffective.