Which of the following bacteria is Cephalexin most effective against?

Cephalexin is a first-generation cephalosporin antibiotic that demonstrates strong activity against gram-positive bacteria, particularly Staphylococcus species. It is particularly effective in treating infections caused by strains of Staphylococcus aureus and Staphylococcus epidermidis. These bacteria are often responsible for skin infections, respiratory tract infections, and other common infections.

The effectiveness of Cephalexin against Staphylococcus is due to its ability to inhibit bacterial cell wall synthesis, a mechanism that is particularly effective against these bacteria that are susceptible to this type of antibiotic.

In contrast, other bacteria listed—such as Klebsiella, Pseudomonas, and Mycobacterium—are less susceptible to Cephalexin. Klebsiella, for instance, is a gram-negative bacterium, and while it can be susceptible to cephalosporins, first-generation agents like Cephalexin are not typically the best choice. Pseudomonas aeruginosa is especially resistant to many antibiotics, including Cephalexin. Lastly, Mycobacterium, which includes the bacteria responsible for tuberculosis, requires specific anti-tubercular agents and is not adequately treated with Cephalexin.