Growth inhibition of Neisseria gonorrhoeae isolates by L-phenylalanine and its analogues in defined media

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Abstract
Growth inhibition by phenylalanine (0.25 mmol/l in defined agar media) was present in .apprx. 1% of over 1000 clinical isolates of N. gonorrhoeae isolates tested. Turbidometry of several phenylalanine-sensitive isolates showed that their growth rates decreased in proportion to phenylalanine concentrations up to about 1 mmol/l. The growth rate was unaffected if 0.04 mmol/l tyrosine was also present. The phenylalanine analog DL-3-fluorophenylalanine inhibited the growth of all 23 isolates further tested on agar. This inhibition was derepressed by phenylalanine in all 17 phenylalanine-resistant isolates. Phenylalanine plus tyrosine were required to derepress the analog inhibition in the other 6 phenylalanine-sensitive isolates. Phenylalanine-sensitive isolates may have a defect in aromatic amino acid synthesis, not involving auxotrophy, but manifested through regulation of the pathways. Phenylalanine effectively repressed tyrosine and phenylalanine synthesis. In 125 isolates including 85 .beta.-lactamase producers (PPNG) and 32 phenylalanine-sensitive isolates, phenylalanine inhibited 63.2% of 38 PPNG isolates carrying the 3.2 megadalton (Md) plasmid, but only one of 47 PPNG isolates carrying the 4.5 Md plasmid. PPNG isolates are most often of the proline ornithine or nonrequiring auxotypes. Phenylalanine sensitivity did not appear to be auxotype dependent.