Ian Chopra

Organic Acids: Chemistry, Antibacterial Activity and Practical Applications

Publisher Summary This chapter presents the chemistry and antimicrobial activity of the saturated straight-chain monocarboxylic acids, and reference is made, where appropriate, to derivatives of this group— for example, unsaturated (cinnamic, sorbic), hydroxylic (citric. lactic), phenolic (benzoic, cinnamic, salicylic) and multicarboxylic (azelaic, citric, succinic) acids. Organic acids are distinguished from other acids by the functional group COOH to which an organic group or a hydrogen atom may be attached. Common names used to describe this group of organic compounds include fatty, volatile fatty, lipophilic, weak, or carboxylic acids. There is no standard protocol for studying the antimicrobial activity of organic acids and many reports give only the pH value of the solution under test without indicating the concentration of the acid, thus making evaluation of the results difficult or impossible. In addition, it is difficult to compare the activity of different acids because it is influenced by the physical chemistry of the microbial species, the growth conditions, and the phase of growth. The chapter discusses two aspects of the use of organic acids: in animal husbandry as animal feed additives and in abattoirs and food-processing plants where they may be used in controlling microbial contamination of carcass meat.

Plasmid-mediated tetracycline resistance in Escherichia coli involves increased efflux of the antibiotic.

Summary The possibility that decreased accumulation of tetracycline by plasmid-carrying strains of Escherichia coli involves increased efflux as well as reduced influx was investigated. Both decreased uptake and increased efflux of tetracycline occurred in resistant strains under conditions where bacteria were still metabolically functional.

Transport of Antibiotics into Bacteria

Publisher Summary This chapter reviews that accumulation of antibiotics by bacteria has, in the past, appeared to be a paradox because it was believed that few drugs could use transport systems involved in uptake of essential solutes. Several examples of antibiotics, which behave in this manner have been described. The situation is not only confined to antibiotics, because various heavy metal ions are also accumulated by systems that usually function to transport essential ions. The potentiation of fosfomycin activity in infected mice by coadministration of glucose 6-phosphate is demonstrated in the chapter. This resulted in induction of uhpT in the bacteria within the animal and increased transport of drug into the microorganisms. Another promising approach involves transport of antibiotics linked to natural substrates, so that the transport system is misused by the adduct. Although some adducts may have chemotherapeutic potential, other hybrid molecules may prove ineffective because they will become too bulky for transport. This may be particularly critical for Gram-negative bacteria where the exclusion limit for passive diffusion through outer membrane porins is usually about 600 daltons.

The Antibacterial Effects of Low Concentrations of Antibiotics

Publisher Summary The effects of sublethal concentrations of antibiotics on bacteria are varied and for the most part uncharacterized at the molecular level. Low concentrations of antibiotics have antimicrobial activity and may also exhibit immunological side effects in the host by either directly inhibiting or stimulating immune-response mechanisms. Exposure of bacteria to low concentrations of β-lactam antibiotics usually leads to growth inhibition accompanied by the production of abnormal morphological forms. However, this is not always so as reports are now emerging to show that certain β-lactam antibiotics are capable either of inducing the synthesis of novel penicillin-binding proteins (PBPs), or altering the balance of existing PBPs to permit normal growth in the presence of β-lactam antibiotics. Despite gaps in the understanding of the molecular basis of the changes, data are accumulating showing that low-level, or single-dose, antibiotic administration can be used for the successful therapy of certain bacterial infections.

A variety of Staphylococcal plasmids present as multiple copies.

In enteric bacteria estimation of the number of plasmids per chromosome reveals two distinct situations (Clowes, 1972): first, when the number of copies is approximately one; and second, when the number of copies is much greater (generally between 7 and 40). This division may also exist for staphylococcal plasmids (Novick & Bouanchaud, 1971), but to test this possibility more fully we have determined the number of plasmid copies per organism for a range of staphylococcal plasmids. We have found that plasmids detectable as covalently closed circular (CCC) DNA are present in multiple copies, i.e. as at least four copies per organism.

Evidence for Mutation to Streptomycin Resistance in Clinical Strains of Staphylococcus aureus

SUMMARY: In three clinical strains of Staphylococcus aureus and one laboratory mutant (609 str-r) the gene determining streptomycin resistance was probably at a single chromosomal locus. Cell-free systems from these, and an additional clinical strain, showed little inhibition of protein synthesis in the presence of streptomycin, whereas systems from sensitive staphylococci were markedly affected by streptomycin.

Mutations Affecting Penicillin-binding Proteins 2a, 2b and 3 in Bacillus subtilis Alter Cell Shape and Peptidoglycan Metabolism

Bacillus subtilis mutants with altered penicillin-binding proteins (PBPs), or altered expression of PBPs, were isolated by screening for changes in susceptibility to beta-lactam antibiotics. Mutations affecting only PBPs 2a, 2b and 3 were isolated. Cell shape and peptidoglycan metabolism were examined in representative mutants. Cells of a PBP 2a mutant (UB8521) were usually twisted whereas PBP 2b (UB8524) and 3 (UB8525) mutants produced helices, particularly after growth at 41 degrees C. The PBP 2a mutant (UB8521) had a higher peptidoglycan synthetic activity than its parent strain whereas the opposite applied to the PBP 2b mutant UB8524. The PBP 3 mutant (UB8525) had a similar peptidoglycan synthetic activity to that of the parent strain when grown at 37 degrees C, but 40% higher activity after growth at 41 degrees C. The PBP 2a mutant (UB8521) exhibited the same wall thickening activity as the parent, but the PBP 2b and 3 mutants (UB8524 and UB8525) were partially defective in this respect. The changes in the susceptibility of PBP 2a, 2b and 3 mutants to beta-lactam antibiotics imply that these PBPs are killing targets, consistent with the fact that these PBPs are also important for shape determination and peptidoglycan synthesis.

Tetracycline Resistance Determinants from Groups A to D Vary in Their Ability to Confer Decreased Accumulation of Tetracycline Derivatives by Escherichia coli

The ability of four genetically distinct plasmid-located tetracycline resistance determinants (TetA, B, C and D) to confer decreased accumulation of tetracycline and some of its analogues by Escherichia coli K12 was examined. Accumulation of oxytetracycline, tetracycline, demethylchlorotetracycline, 6-demethyl-6-deoxy-5-hydroxy-6-methylene-tetracycline, chlorotetracycline, doxycycline and 6-demethyl-6-deoxytetracycline was examined by fluorescence spectroscopy. The determinants varied in their ability to promote decreased accumulation of tetracyclines, defined as an R+/R- fluorescence ratio of less than 0.85. Plasmid pIP7 (TetA) caused reduced accumulation of only oxytetracycline, tetracycline and chlorotetracycline, but plasmid pDU301 (TetB) promoted reduced accumulation of all the compounds tested except 6-demethyl-6-deoxytetracycline. The TetC determinant of pBR322 caused decreased uptake of five derivatives, but not doxycycline or 6-demethyl-6-deoxytetracycline. Plasmid RA1 (TetD) encoded reduced accumulation of oxytetracycline, tetracycline, 6-demethyl-6-deoxy-5-hydroxy-6-methylenetetracycline and chlorotetracycline. In general, the resistance determinants were more efficient in promoting decreased accumulation of hydrophilic tetracyclines. These accumulation studies provide a satisfactory method for the phenotypic identification of Tet resistance determinants.

Prediction of signal sequence-dependent protein translocation in bacteria: Assessment of the Escherichia coli minicell system

The use of phenethyl alcohol (PEA) as a probe for signal sequence-dependent protein translocation in minicells was examined. Processing of beta-lactamases and tonA was inhibited by PEA at concentrations which did not affect production of the alpha and gamma forms of penicillin binding protein (PBP) lb. The PBPlbs are believed to lack leader sequences whereas the other proteins contain them. Processing of a beta-lactamase which shares the murein-lipoprotein export pathway was relatively resistant to PEA, consistent with previous findings in whole bacteria. The results reported here suggest that PEA is a suitable probe for leader sequences in the minicell system. By using PEA we predict that PBP4 does not require a leader sequence for membrane insertion.

Chapter 15. Transport of Antibiotics into Bacteria

Publisher Summary This chapter discusses the recent researches on transport of antibiotics into bacteria. The aminoglycoside-aminocyclitol (AGAC), a group of antibiotics, comprises a large number of clinically useful drugs that are able to enter both gram-positive and gram-negative bacteria to interfere with protein synthesis. The beta-lactams antibiotics are located on the surface of bacterial cytoplasmic membranes. Therefore, in gram-negative organisms, these antibiotics must cross the bacterial outer membrane to exert their inhibitory effects. The majority of beta-lactams cross the outer membrane by passive diffusion through porin channels. Pseudomonas aeruginosa is intrinsically resistant to most beta-lactams, a situation resulting from poor uptake of drug molecules, across the outer membrane. On the other hand the cephalosporin E-0702 and other catechol bearing structures are transported across the bacterial outer membrane by the tonB-dependent iron transport system. Structure-activity studies have showed that the tonB-dependent transport system is relatively tolerant of chemical variability in the substrate. Transport of mupirocin antibiotic into B. subtilis and S. aureus has been recently investigated. Most quinolones are low molecular-weight hydrophilic molecules. These properties imply that the drugs could well cross the outer membrane through porin channels. Quinolones are indeed known to penetrate the outer membrane of E. coli through OmpF and OmpC porins. The design of novel antibacterial agents that can utilize existing bacterial transport systems for entry into bacteria is an attractive prospect for the therapy of infectious diseases.