Gentamicin in combination with -lactam antibiotics (such as penicillin or cephalosporins) usually ACTS synergically against a variety of gram-negative bacteria, including Pseudomonas aeruginosa.It also has a synergistic effect on gram-positive bacteria.For example, gentamicin ACTS on streptococcus with penicillin, staphylococcus aureus with enzyme resistant semisynthetic penicillin, pneumococcus pneumoniae with cephalosporin, listeria with penicillin G or ampicillin, pseudomonas aeruginosa with carbopicillin and so on.This is because beta-lactams cause damage to the bacterial cell wall, which increases the uptake of gentamicin by the bacterial cell membrane, because the damage to the cell wall makes the drug accessible to the cell membrane.