Microemulsions are physically stable oil/water systems that have potential use as delivery systems for many pharmaceuticals which are normally of limited use due to their hydrophobicity, toxicity or inability to access the site of action. It has been suggested that microemulsions are self‐preserving antimicrobials in their own right, although there is little evidence to support this. In this experiment, microemulsions of various compositions were formulated and tested for their stability and antimicrobial action. The physical stability of the different microemulsions was assessed by centrifugation at 4000 g and by storage in a water bath at 37 °C for one month, during which no phase separation was observed. The antimicrobial activity of the microemulsions was tested using the compendial method, observation of the kinetics of killing, and transmission electron microscopy (TEM) of microemulsion‐exposed cultures of Pseudomonas aeruginosa PA01. These latter experiments on Ps. aeruginosa indicated distinct signs of membrane disruption. The results indicated that the microemulsions are self‐preserved, and that their killing of microbial cultures is very rapid and may be the result of membrane activity.

Benzisothiazolone (BIT), N‐methylisothiazolone (MIT) and 5‐chloro‐N‐methylisothiazolone (CMIT) are highly effective biocidal agents and are used as preservatives in a variety of cosmetic preparations. The isothiazolones have proven efficacy against many fungal and bacterial species including Pseudomonas aeruginosa. However, some species are beginning to exhibit resistance towards this group of compounds after extended exposure. This experiment induced resistance in cultures of Ps. aeruginosa exposed to incrementally increasing sub‐minimum inhibitory concentrations (MICs) of the isothiazolones in their pure chemical forms. The induced resistance was observed as a gradual increase in MIC with each new passage. The MICs for all three test isothiazolones and a thiol‐interactive control compound (thiomersal) increased by approximately twofold during the course of the experiment. The onset of resistance was also observed by reference to the altered presence of an outer membrane protein, designated the T‐OMP, in SDS‐PAGE preparations. T‐OMP was observed to disappear from the biocide‐exposed preparations and reappear when the resistance‐induced cultures were passaged in the absence of biocide. This reappearance of T‐OMP was not accompanied by a complete reversal of induced resistance, but by a small decrease in MIC. The induction of resistance towards one biocide resulted in the development of cross‐resistance towards other members of the group and the control, thiomersal. It has been suggested that the disappearance of T‐OMP from these preparations is associated with the onset of resistance to the isothiazolones in their Kathon™ form (CMIT and MIT).

Chitosan is a plentiful organic polysaccharide, which can be relatively easily obtained by chemical modification of animal or fungal source materials. Chitosan and its derivatives have been shown to exhibit direct anti-viral activity, to be useful vaccine adjuvants and to have potential anti-SARS-CoV-2 activity. This thorough and timely review looks at the recent history of investigations into the role of chitosan and its derivatives as an antiviral agent and proposes a future application in the treatment of endemic SARS-CoV-2.

Aims:  The effect of subminimal inhibitory concentrations (sub‐MICs) of cefalexin, ciprofloxacin and roxithromycin was investigated on some virulence factors [e.g. coagulase, Toxic Shock Syndrome Toxin 1 (TSST‐1) and biofilm formation] expressed by Staphylococcus aureus biofilms. Methods and Results:  Biofilms were grown with and without the presence of 1/16 MIC of antibiotics on Sorbarod filters. Eluate supernatants were collected, and coagulase and TSST‐1 production were evaluated. Coagulase production was reduced in eluates exposed to roxithromycin when compared to control, while TSST‐1 production was reduced in biofilms exposed to cefalexin and to a lesser extent, ciprofloxacin. In addition, the ability of Staph. aureus to produce biofilm in microtitre plates in the presence of sub‐MIC antibiotics indicated that cefalexin induced biofilm formation at a wide range of sub‐MICs. TSST‐1 produced from the challenged and control biofilms was purified, and its proliferative activity was studied on single cell suspension of mouse splenocytes using MTS/PMS assay. No significant difference in the activity between the treated toxin and the control has been observed. Conclusions:  Antibiotics at sub‐MIC levels interfere with bacterial biofilm virulence expression depending on the type and concentration of antibiotic used. Significance and Impact of the Study:  The establishment of sub‐MICs of antibiotics in clinical situations may result in altered virulence states in pathogenic bacteria.

Aims: The demonstration of the antibiofilm effects of pharmaceutical microemulsions. Methods and Results: Microemulsions were prepared as physically stable oil ⁄ water systems. Previous work by this group has shown that microemulsions are highly effective antimembrane agents that result in rapid losses of viability in planktonic populations of Pseudomonas aeruginosa and Staphylococcus aureus. In this experiment a microemulsion preparation was used upon established biofilm cultures of Ps. aeruginosa PA01 for a period of 4 h. The planktonic MIC of sodium pyrithione and the planktonic and biofilm MICs of cetrimide were used as positive controls and a biofilm was exposed to a volume of normal sterile saline as a treatment (negative) control. Results indicate three log-cycle reductions in viability within the microemulsion treated biofilm, as compared to those observed in control treatments of similar biofilms (one log-cycle reduction in viabilities). Conclusions:The results indicate that the microemulsions are highly effective antibiofilm agents. Significance and Impact of the Study: This study suggests that microemulsions may have a role in the treatment of industrial and environmental biofilms.