Inducible Resistance to Tetracycline in Staphylococcus aureus

Abstract

S U M M A R YWhen staphylococci, resistant to 4 pg. tetracycline/ml., were grown in nutrient media at subinhibitory levels of the drug, phenotypical resistance increased until the cocci grew with 160 pg. tetracyclinelml. Resistance increased most rapidly at the highest concentration of tetracycline which did not significantly inhibit growth. Increase in resistance was also obtained by pre-incubation with p-apo-5-oxy-tetracycline. Increase in resistance could be prevented by chloramphenicol and actinomycin D, but not by nalidixic acid. When a highly resistant culture was transferred to tetracycline-free medium, phenotypical resistance decreased gradually ; after four transfers on nutrient agar it returned entirely to the original level. I N T R O D U C T I O NWilliams (1967) has drawn attention to the resistance to mercury salts (Moore, I 960) of tetracycline-resistant strains of Staphylococcus aureus. Resistance to tetracycline was in general correlated with resistance to mercury; in mercury-sensitive strains, however, high-level resistance to tetracycline could be obtained by ' training ', i.e. growth in peptone water containing 3 pg. antibiotic/ml. METHODSBacterial strains. The strain studied, Staphjdococcus aureus I I I (Sompolinsky, Yiflah & Aboud, 1568), was susceptible to mercuiy salts (Moore, 1960) and resistant to tetracycline (Tc) by virtue of a plasmid that could be eliminated by growth at 44" (May, Houghton & Perret, 1964). The eliminated susceptible strain grew on nutrient agar containing 0.2 pg. Tc/ml. ; on agar containing 0.4 pg. Tc/ml., colonies did not develop from an inoculum of IO* staphylococci. With the uneliminated culture, identical colony counts were obtained on plain agar and on agar containing 4-0 pg. Tc/ml. At concentrations of up to 160pg. Tc/ml., some colonies developed but growth did not occur on nutrient agar containing 320pg. Tc/ml. Three types of Iesistant cultures have been used: (a) a culture transferred daily on plain nutrient agar; only 5 to 10% of the viable cocci from this culture produced colonies on agar containing 10 pg. Tc/ml. ; (b) a highly resistant culture subcultured daily on nutrient agar containing I 60 pg. Tc/ml.; and (c) cultures with intermediate levels of resistance obtained by growth of type (a) culture for a short period in nutrient broth containing tetracycline at different concentrations.

Biochemical and Genetic Basis of Tetracycline Resistance in Staphylococcus aureus

“…The macro-inducible tetracycline-resistant staphylococci described here appear to be typical of strains possessing wild-type plasmids (34), but neither macro-constitutive types, in which the genes specifying resistance are chromosomal, nor micro-constitutive strains, in which resistance is plasmid mediated, have previously been described.…”

“…on the syntheses of ribonucleic acid and protein, this drug may be used to distinguish between selection of resistant organisms and induction of resistance involving repressor-operator structural gene interactions (34). Overnight cultures were diluted into fresh warmed broth and grown with aeration to culture densities of 2 x 108 organisms per ml.…”

“…However, many aspects of both the genetic and biochemical basis of this resistance in Staphylococcus aureus and Escherichia coli still require clarification. In both organisms, maximal phenotypic expression of resistance usually only occurs after induction with subinhibitory concentrations of tetracycline (7,8,14,34). The instability of tetracycline resistance in some strains of S. aureus at 44 C suggested that the genes coding for resistance were specified by plasmids (2,26); a plasmid inheritance for this resistance was confirmed by the isolation of plasmid-specific deoxyribonucleic acid (DNA) (4,21,30).…”

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“…One of the bacteriostatic antibiotics, tetracycline (TC), is a specific inhibitor of protein synthesis in vivo as well as in vitro (1,12,13). Studies carried out in several laboratories have shown that, unlike other types of bacterial resistance, TC resistance is not due to either the modification of ribosome(s) and of protein synthetic machinery or of enzymatic destruction of the drug (3,8,10).…”

Antibacterial and Inducer Activities for Tetracycline Resistance by Its Derivatives and Analogues

“…In strains with inducible TC resistance, the mechanism of the resistance has been considered to be a specific decrease of permeability of the bacterial membrane (2,4,7,13). Recent studies by curing experiments and transduction to a rec-recipient revealed that almost all genetic determinants governing TC resistance in staphylococci are located on a plasmid (S. Mitsuhashi and M. Inoue, U.S.-Japan Scientific Seminar on Bacterial Plasmid, Hawaii, June 1972).…”