This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
[{'content': 'All of the following statements are true regarding Staphylococci except - \nSelect and explain. \nA. A majority of infections caused by coagulase-negative Staphylococci are due to Staphylococcus epidermidis, B. β-lactamase production in Staphylococci is under plasmid control, C. Expression of methicillin resistance in Staphylococcus aureus increases when it is incubated at 37°C on blood agar., D. Methicillin resistance in Staphylococcus aureus is independent of β-lactamase production', 'role': 'user'}, {'content': 'Answer: C \nExplanation: "Methicillin resistance is expressed more at 30°C than at 37°C", so susceptibility studies for methicillin are routinely performed at reduced temperature (30° - 35° C for 24 hrs), with increased concentration of salt in the medium to enhance the expression of resistance.\nResistance to antibiotics \n\nPenicillin resistance is of three types-\n\n1) Production of beta-lactamase (penicillinase)\n\nIt inactivates penicillin by splitting β-lactam ring\nIts production is controlled by plasmid.\nPlasmid is transmitted by transduction (mainly) or conjugation.\nPenicillinase is an inducible enzyme\n\n\nAs a result of the widespread dissemination of plasmids containing penicillinase, less than 5% strains of staph. remain susceptible to penicillin.\nStaphylococci produce four type of penicillinases, A to D. Hospital strains usually forms type \'A\' penicillinase.\n\n2) Changes in bacterial surface receptors by lack or inaccessibility of certain penicillin binding proteins (PBPs) in the organism.\n\nMethicillin resistant is mainly due to this mechanism and is independent of β - lactamase production.\nIt is due to production of PBP 2a.\nThis change (in bacterial surface receptors) is normally chromosomal in nature.\nThe resistance gene is mec A gene which is a part of large mobile genetic element - staphylococcal cassette chromosome (SCCmec).\nThis genetic material has been transferred to Staph. aureus from S. seiuri.\n\n3) Development of tolerance to penicillin, by which the bacterium is only inhibited but not killed.\nAbout option \'a\'\n"Approximately 75% of the infections caused by coagulase negative staphylococci are due to staphylococcus\xa0epidertnidis" -', 'role': 'assistant'}]