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The strain of bacteria recognized for its resistance to all penicillin is MRSA, which stands for Methicillin-Resistant Staphylococcus aureus. This specific strain of Staphylococcus aureus has mutations that confer resistance not only to methicillin but also to all beta-lactam antibiotics, including penicillins. The presence of the mecA gene in MRSA enables the bacteria to produce a penicillin-binding protein that penicillins cannot bind to, rendering these antibiotics ineffective.

That’s why MRSA is particularly concerning in clinical settings, as it limits the options for treatment significantly and can lead to severe infections that are difficult to manage. Other strains mentioned do not exhibit the same level of resistance to all penicillins. For instance, while Staphylococcus aureus includes strains that might be susceptible to some forms of penicillin, MRSA is specifically identified for its resistant properties.