In Mueller-Hinton agar, what can cause increased resistance of Pseudomonas aeruginosa to aminoglycosides?

Increased resistance of Pseudomonas aeruginosa to aminoglycosides in Mueller-Hinton agar can be attributed to elevated levels of calcium (Ca++) and magnesium (Mg++). These divalent cations play a significant role in the permeability of bacterial membranes and can enhance the stability of the outer membrane in Gram-negative bacteria, including Pseudomonas aeruginosa.

When the concentrations of Ca++ and Mg++ are increased, they can help to stabilize the anionic components of the bacterial cell wall, leading to reduced uptake of aminoglycosides. Aminoglycosides require transport across the inner membrane to exert their antibacterial effects, and the presence of these cations can create a barrier against the entry of these drugs, thus contributing to a higher level of resistance.

Understanding the role of cations in bacterial resistance mechanisms is crucial for both laboratory and clinical settings, especially for addressing infections caused by Pseudomonas aeruginosa, which is known for its intrinsic resistance to many antibiotics.