Non-inherited antibiotic resistance

In addition to their impressive, well-publicized and well-researched propensity to evolve and acquire genetically determined mechanisms for resistance to antibiotics, bacteria that are inherently susceptible to these drugs can also be phenotypically refractory to their action. This phenomenon of 'non-inherited resistance' to antibiotics has been known since the beginning of the antibiotic era but, relative to inherited resistance, it has been given little attention. Here, we review the in vitro and in vivo evidence for the different forms of non-inherited resistance and the mechanisms responsible. With the aid of a simple mathematical model and computer simulations, we show how non-inherited resistance could extend the duration of antibiotic treatment, cause treatment failure and promote the generation and ascent of inherited resistance in treated patients.     

Mammalian antibiotic peptides

The increasing development of bacterial resistance to traditional antibiotics has reached alarming levels, thus creating a strong need to develop new antimicrobial agents. These new antibiotics should possess novel mechanisms of action and different cellular targets compared with existing antimicrobials. Recent discoveries and isolations of so-called animal antibiotics, mostly small cationic peptides, which represent a potent branch of natural immunity, offered the possibility to acquire new and effective antibiotics of this provenance. To this date, more than 500 antibiotic peptides have been distinguished and defined. Their antimicrobial properties present new opportunities for their use as antibiotics or for construction of their more effective derivatives, but much research is still required to pave the way to their practical use. This is a survey of substances forming an armamentarium of natural immunity of mammals.     

Tackling antibiotic resistance

The development and spread of antibiotic resistance in bacteria is a universal threat to both humans and animals that is generally not preventable but can nevertheless be controlled, and it must be tackled in the most effective ways possible. To explore how the problem of antibiotic resistance might best be addressed, a group of 30 scientists from academia and industry gathered at the Banbury Conference Centre in Cold Spring Harbor, New York, USA, from 16 to 18 May 2011. From these discussions there emerged a priority list of steps that need to be taken to resolve this global crisis.     

Predicting antibiotic resistance

The treatment of bacterial infections is increasingly complicated because microorganisms can develop resistance to antimicrobial agents. This article discusses the information that is required to predict when antibiotic resistance is likely to emerge in a bacterial population. Indeed, the development of the conceptual and methodological tools required for this type of prediction represents an important goal for microbiological research. To this end, we propose the establishment of methodological guidelines that will allow researchers to predict the emergence of resistance to a new antibiotic before its clinical introduction.     

Static recipient cells as reservoirs of antibiotic resistance during antibiotic therapy

How does taking the full course of antibiotics prevent antibiotic resistant bacteria establishing in patients? We address this question by testing the possibility that horizontal/lateral gene transfer (HGT) is critical for the accumulation of the antibiotic-resistance phenotype while bacteria are under antibiotic stress. Most antibiotics prevent bacterial reproduction, some by preventing de novo gene expression. Nevertheless, in some cases and at some concentrations, the effects of most antibiotics on gene expression may not be irreversible. If the stress is removed before the bacteria are cleared from the patients by normal turnover, gene expression restarts, converting the residual population to phenotypic resistance. Using mathematical models we investigate how static recipients of resistance genes carried by plasmids accumulate resistance genes, and how specifically an environment cycling between presence and absence of the antibiotic uniquely favors the evolution of horizontally mobile resistance genes. We found that the presence of static recipients can substantially increase the persistence of the plasmid and that this effect is most pronounced when the cost of carriage of the plasmid decreases the cell's growth rate by as much as a half or more. In addition, plasmid persistence can be enhanced even when conjugation rates are as low as half the rate required for the plasmid to persist as a parasite on its own.     

Direction of aminoacylated transfer RNAs into antibiotic synthesis and peptidoglycan-mediated antibiotic resistance

Prokaryotic aminoacylated-transfer RNAs often need to be efficiently segregated between translation and other cellular biosynthetic pathways. Many clinically relevant bacteria, including Streptococcus pneumoniae, Staphylococcus aureus, Enterococcus faecalis and Pseudomonas aeruginosa direct some aminoacylated-tRNA species into peptidoglycan biosynthesis and/or membrane phospholipid modification. Subsequent indirect peptidoglycan cross-linkage or change in membrane permeability is often a prerequisite for high-level antibiotic resistance. In Streptomycetes, aminoacylated-tRNA species are used for antibiotic synthesis as well as antibiotic resistance. The direction of coding aminoacylated-tRNA molecules away from translation and into antibiotic resistance and synthesis pathways are discussed in this review.     

Roflamycoin--a new channel-forming antibiotic

Ion permeability of lipid bilayers was studied in the presence of a new antifungal pentaene antibiotic, roflamycoin, the structure of which differs considerably from that of the well-known polyene channel-former amphotericin B. Both of them, however, show the property of increasing the membrane permeability only in the case of sterol-containing membrane when added on both its sides. The conductance is strongly dependent on the concentration of the antibiotic in the solutions and of sterol in the membrane. Unlike the amphotericin B channels, roflamycoin channels are potential-dependent and have short lifetime (approx. 1 s) and high conductance (approx. 100 ps in 1 M KCl), which increases linearly with the salt concentration and is not blocked by the familiar blockers of amphotericin B channels. The two antibiotics seem to have a common mechanism of channel formation, viz. the formation starts from two semi-pores assembled in the opposite monolayers from several molecules of the antibiotic and sterol. However, the inner diameter of the roflamycoin channel is larger because of the different antibiotic-to-sterol ratio in the channel aggregate. It is believed that the difference in the ratio is due to the presence of the methyl group in the polyene chain of roflamycoin, and the considerable difference in lifetimes of the two types of channels depends on the terminal groups of the antibiotics.     

Ultrathin antibiotic walled microcapsules

Ultrathin microcapsules comprised of anionic polyelectrolytes (PE) and a polycationic aminoglycoside (AmG) antibiotic drug were prepared by depositing PE/AmG multilayers on zinc oxide (ZnO) colloid particles using the layer-by-layer self-assembly technique and subsequently dissolving the ZnO templated cores. The polyelectrolytes, dextran sulfate sodium (DxS) and poly(styrenesulfonate) (PSS), were selected owing to their different backbone structure. An aminoglycoside, tobramycin sulfate (TbS), was used for studying DxS/TbS or PSS/TbS multilayer films. The multilayer growth on ZnO cores was characterized by alternating zeta potential values that were different for the DxS/TbS and PSS/TbS multilayers due to the PE chemistry and its interaction with Zn(2+) ions. Transmission and scanning electron microscopy provide evidence of PE/TbS multilayer coating on ZnO core particles. The slow acid-decomposition of the ZnO cores using weak organic acids and the presence of sufficient quantity of Zn(2+) in the dispersion were required to produce antibiotic multilayer capsules. There was no difference in the morphological characteristics of the two types of capsules; although, the yield for [PSS/TbS](5) capsules was significantly higher than for [DxS/TbS](5) capsules which was related to the physicochemical properties of DxS/TbS/Zn(2+) and PSS/TbS/Zn(2+) complexes forming the capsule wall. The TbS quantity in the multilayer films was determined using a quartz crystal microbalance and high performance liquid chromatography techniques which showed less TbS loading in both, capsules and multilayers on planar gold substrate, than the theoretical DxS:TbS or PSS:TbS stoichiometric ratio. The decomposition of the [PE/TbS](6) multilayers was fastest in physiological buffer followed by mannitol and water. The decomposition rate of the [PSS/TbS](6) multilayers was slower than [DxS/TbS](6) monolayers. The incomplete decomposition of DxS/TbS under saline conditions suggests the major role of hydrogen bonding for stability of DxS/TbS multilayers. A combination of hydrogen bonding and hydrophobic interaction between phenyl rings in PSS was responsible for PSS/TbS multilayer stability. In vivo studies in rabbits highlight the safety and sustained drug delivery potential of the PE/AmG microcapsules. The antibiotic walled ultrathin capsules presented here are suitable for sustained ophthalmic antibiotic delivery.     

Surveillance of antibiotic resistance

Surveillance involves the collection and analysis of data for the detection and monitoring of threats to public health. Surveillance should also inform as to the epidemiology of the threat and its burden in the population. A further key component of surveillance is the timely feedback of data to stakeholders with a view to generating action aimed at reducing or preventing the public health threat being monitored. Surveillance of antibiotic resistance involves the collection of antibiotic susceptibility test results undertaken by microbiology laboratories on bacteria isolated from clinical samples sent for investigation. Correlation of these data with demographic and clinical data for the patient populations from whom the pathogens were isolated gives insight into the underlying epidemiology and facilitates the formulation of rational interventions aimed at reducing the burden of resistance. This article describes a range of surveillance activities that have been undertaken in the UK over a number of years, together with current interventions being implemented. These activities are not only of national importance but form part of the international response to the global threat posed by antibiotic resistance.     

Dynamics of antibiotic resistant Mycobacterium tuberculosis during long-term infection and antibiotic treatment

For an infecting bacterium the human body provides several potential ecological niches with both internally (e.g. host immunity) and externally (e.g. antibiotic use) imposed growth restrictions that are expected to drive adaptive evolution in the bacterium, including the development of antibiotic resistance. To determine the extent and pattern of heterogeneity generated in a bacterial population during long-term antibiotic treatment, we examined in a monoclonal Mycobacterium tuberculosis infection antibiotic resistant mutants isolated from one patient during a 9-years period. There was a progressive accumulation of resistance mutations in the infecting clone. Furthermore, apparent clonal sweeps as well as co-existence of different resistant mutants were observed during this time, demonstrating that during treatment there is a high degree of dynamics in the bacterial population. These findings have important implications for diagnostics and treatment of drug resistant tuberculosis infections.