The Effect of Nutrient Limitation on the Drug Resistance of Proteus Species

Abstract

A typical polymyxin-resistant Proteus mirabilis, NCTC 5887, was compared with a polymyxin-sensitive clinical isolate, RBH. Their quantitative nutritional requirements were found to be the same. A minimal medium was formulated. For sensitivity tests, an excess of KCl was added, because these strains, and other strains tested, formed stable small-colony variants, with drug resistance patterns different from the original organisms. Variants grew readily in the minimal medium, but relatively slowly at high KCl concentrations.

Stationary-phase cultures induced by glucose-depletion (C-dep) were compared with cultures depleted of Mg²⁺ (Mg-dep) or phosphate (P-dep) before being glucose-depleted. Mg-dep and P-dep RBH were more resistant to polymyxin than C-dep. Although the minimum inhibitory concentrations of cetrimide, chlorhexidine and phenol were almost the same for these strains, the sensitivities of cells after nutrient depletion were very different. For example, the sensitivity to cetrimide of Mg-dep RBH > C-dep > P-dep, but for 5887 C-dep > Mg-dep and P-dep. Results were also modified by the constituents of the test menstruum. Using chemostat cultures, faster-growing Proteus were more sensitive to polymyxin than slow growing ones.

5887 contained twice the 2-keto-3-deoxyoctonic acid found in RBH, and its cell walls contained more phospholipid. Mg-dep RBH cell walls contained more phospholipid than C-dep, and the ratios of individual phospholipids were changed. These results support the hypothesis that polymyxin binds to lipopolysaccharide and/or phospholipid in the outer membrane, resistance depending on a reduction in the amount of polymyxin reaching the inner membrane. No evidence was found that cell wall cations were related to resistance. The variations in drug sensitivity and cell wall chemistry demonstrate the need for defined growth conditions and reproducible inocula for the microbiological assay of antibiotics and other antibacterial agents.

Date of Award	1979
Original language	English
Awarding Institution	Aston University