The Effect of Growth Conditions on B-lactam Resistance in Enterobacter Cloacae

Abstract

The nutritional requirements of Enterobacter cloacae E7 (a clinical isolate) were quantitatively defined using a chemically defined medium (CDM). E.cloacae strains E12, E13,E14 and E16 (also clinical isolates) grew in this medium at similar growth rates to E7. Antibiotic sensitivity patterns were determined for these five strains of E.cloacae. Bacteria grown in Iso-sensitest broth, urine and serum were generally 2-8 fold less sensitive than those grown in nutrient deprived CDM. The effect of growth medium and rate on β-lactamase production and on induction of B-lactamase by cefotaxime was examined. Growth medium and rate had no significant effectupon β-lactamase production in the absence of β-lactam antibiotics. However growth rate profoundly affected β-lactamase induction by cefotaxime. Faster growing bacteria produced greater levels of β-lactamase activity insignificantly less time than slower growing bacteria. Expression of porin proteins and iron regulated membrane proteins (IRMPs), in different media, was subject to variation both within and between strains of E.cloacae. However there was no correlation between expression of these outer membrane proteins and growth medium, nor was there any correlation between expression of porin proteins and antibiotic susceptibility. Growth in iron-deprived CDM, urine and serum and growth in phosphate deprived CDM (CDM-P)induced the expression of IRMPs in E.cloacae. Penicillin-binding proteins (PBPs) 5 and 6 of E.cloacae were sensitive to changes in the growth medium, particularly in serum and CDM-P. Further investigations are required to determine the effect of growth medium on the higher molecular weight PBPs.