The use of the antiseptic chlorhexidine as a therapeutic or prophylactic drug in dental or periodontal hygiene is a departure from the usual fields of use for this agent. Therefore, a review of some of its antibacterial properties is desirable. The bacteriostatic spectrum of chlorhexidine * was determined: there was a wide spectrum of activity with gram‐positive cocci being especially sensitive (MIC 0.19 to 2.0 μg/ml). Exposure of suspensions of various bacterial species to chlorhexidine (0.02%) for 10 minutes at room temperature, reduced the viable organisms by about 99.99% in most instances. Addition of serum to the test system reduced the bactericidal action markedly. A study with Streptococcus mutans revealed that the presence of sucrose (5%) in the growth‐medium used for preparing bacterial suspensions, profoundly influenced the bactericidal activity. This is presumed to be the result off adsorption of chlorhexidine to extracellular polysaccharides. Attempts to select Escherichia coli and oral streptococci resistant to chlorhexidine by serial passage in subinhibitory concentrations in vitro, indicated that mutants were rare. In contrast, mutants of E. coli resistant to ampicillin or streptomycin were selected with ease. Predictions of what might happen under clinical conditions (where oral organisms might be exposed to sublethal concentrations for very long periods of time) cannot be made from such laboratory experiments. No changes were observed in the susceptibility of faecal organisms of hamsters given chlorhexidine (50 mg/kg) daily by gastric catheter for 28 days but 100 mg/kg rapidly caused a lethal enteritis which was assumed to be the result of gross disturbances of the alimentary microflora. A small study in three human volunteers is reported. Use of a mouthrinse (twice a day for up to seven weeks) containing chlorhexidine digluconate (0.2%) resulted in a slight but transient change in susceptibility of salivary organisms to the bacteriostatic action of the agent. Whether resistance will prove to be an obstacle to the use of chlorhexidine in prophylaxis, in the mouth can be decided only with more clinical experience.
S U M M A R YProduction of P-lactamase by nine strains of the genus Enterobacter (eight Enterobacter cloacae, one E. aerogenes) was studied to determine its inducibility. Induction was observed with benzylpenicillin (500 ,ag./ml.) in all except one strain. Cultures were examined to locate the enzyme; it was found that in exponential growth the enzyme was cell-bound, and in stationary phase cultures much of it was in the culture medium. Maximum enzyme activity was only demonstrated after cell-breakage. Substrate profiles of crude enzyme preparations were examined and it was observed that the enzymes were 20-80 times more active against cephalosporins than against benzylpenicillin. Evidence is presented which suggests that one strain of E. cloacae produced two /?-lactamases, an inducible cephalosporinase and a constitutive penicilli nase.
117Streptococcus mutans strain Ingbritt, and its derivative B7 which had been passaged through monkeys, have been used to investigate how the synthesis of extracellular glucosyl-and fructosyltransferases is regulated. The most active enzyme from carbon-limited continuous cultures was a fructosyltransferase ; enzymes catalysing the formation of water-insoluble glucans from sucrose were relatively inactive. Dextransucrase (EC 2.4.1 .5), which catalyses soluble glucan synthesis, was most active in the supernatant fluid from cultures grown with excess glucose, fructose or sucrose, but full activity was detected only when the enzyme was incubated with both sucrose and dextran. Little dextransucrase activity was detected in carbon-limited cultures. It is concluded that glucosyl-and fructosyltransferases are constitutive enzymes in that they are synthesized at similar rates during growth with an excess of the substrate or of the products of the reactions which they catalyse. Although the Ingbritt strain was originally isolated from a carious lesion, it is now a poor source of glucosyltransferase activity. Glucosyltransferases were extremely active in cultures of a recent clinical isolate, strain 3209, and were apparently induced during growth with excess glucose.
The key features of the interaction of chlorhexidine with bacteria leading to death are adsorption, damage to permeability barriers and precipitation of the cytoplasm. The bacteriostatic profile indicates that Streptococcus mutans is highly susceptible but it is not known whether bacteriostatic or bactericidal activity is crucial to antiplaque activity. Although some correlation overall was found between bactericidal and bacteriostatic susceptibility amongst more than 80 strains of various species, reference to the minimum inhibitory concentration alone gives no firm indication about the possible lethal action of chlorhexidine. The effects of saliva and pH on antibacterial activity are given. In human studies reported by others, repeated oral application of chlorhexidine reduced the numbers of salivary organisms recoverable and this was accompanied by slight (seemingly clinically insignificant) alterations in the sensitivity of certain salivary organisms. Daily application of a chlorhexidine dental gel in a 6‐week trial did not result in detectable changes of susceptibility of the salivary flora. Rats dosed daily in drinking water (5‐10 mg chlorhexidine base per kg) over a 2‐year period yielded buccal organisms with reduced sensitivity: this was not reflected amongst faecal bacteria. The faecal flora changed quantitatively in a dose‐dependent manner and coliform organisms particularly were reduced in number. Caution is required in interpreting relative sensitivities of different bacterial strains or species from data obtained using agar‐diffusion methods.
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