The Antibiotic Epidemic Antibiotic Resistance

Antibiotic Resistance: Surviving An Uncertain Future

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Maintenance Of Antibiotic Resistance

A variety of surveys have indicated that normal healthy humans (who are not pursuing a course of antibiotic therapy) carry antibiotic-resistant enteric species in their intestinal tract a substantial proportion are found to contain transmissible antibiotic resistance plasmids. Studies have demonstrated that a lack of antibiotic selective pressure, for example, removing antimicrobials from cattle feed, can lead to a gradual decrease in the percentage of resistance genes and resistance bacteria found in a population (Langlois et al., 1986 Hintone et al., 1985).

Multiple antibiotic resistance and mercury resistance

Although the cooccurrence of antibiotic resistance and resistance to heavy metals such as mercury has long been known, its implications for public health are only now becoming clear. In 1964, the cotransduc-tion of genes encoding resistance to penicillin and mercury by a staphylococcal phage was reported (Richmond and John, 1964). Ten years later it was found that 25 of the antibiotic resistance plasmids isolated from enteric bacteria in Hammersmith Hospital also carried mercury resistance (Schottel et al., 1974). DNA sequence analyses has shown that the Tn22-type transposons carry both a copy of the mer locus and an integron (see above) (Stokes and Hall, 1989 Grinsted et al., 1990). Summers and co-workers (1993) have observed that resistance to mercury occurs frequently in human fecal flora and is correlated with the occurrence of multiple antibiotic resistance. How does this phenomenon become a public health concern In the same report Summers' group found that the mercury released...

Genetics Of Antibiotic Resistance By Bacteroides

Antibiotic resistance is an increasing problem in the treatment of Bacteroides infections The bacteria continue to acquire genes that make them resistant to multiple antibiotics. A drastic increase in resistance to antibiotics such as tetracycline, cephalosporins, and clindamycin over the last 2 decades has necessitated the use of carbapenems, metronidazole, and j-lactamase inhibitors. Resistance to these latter agents, however, is also increasing. The genetic elements responsible for the evolution of antibiotic resistance in Bacteroides are the topic of this section. B. Conjugative elements involved in the transfer of antibiotic resistance elements that are self-transmissible. Both conjugative transposons and conjugative plasmids are involved in the transfer of these antibiotic-resistance genes in Bacteroides. Several factors account for the ability of the con-jugative transposons to propagate antibiotic-resistance genes so successfully. Their broad host range allows for their...

Overcoming barriers to transformation

Two T-DNA border sequences that flank various restriction sites for cloning as well as an antibiotic resistance gene to select for transformed plant cells. If the plasmid carries a single border sequence, the entire plasmid is delivered to plants, and recent work indicates that A. tumefaciens can deliver as much as 180-kb of DNA to plants. If the plasmid carries two border sequences, only the DNA bounded by T-DNA borders is delivered to plants. Second, the frequency of stable transformation is often very high, far exceeding frequencies achieved by other gene delivery methods. For example, co-cultivation of A. tumefaciens with regenerating protoplasts of certain plant species can result in transformation of up to one-half of the protoplasts.

General considerations

Although gene exchange may under normal circumstances be rare in stable microbial microcosms, the intense selective pressure of antibiotic usage is likely to have provoked cascades of antibiotic resistance gene transfer between unrelated microbes. These transfers must involve different biochemical mechanisms during which efficiency is not a critical factor since the survival and multiplication of a small number of resistant progeny suffices to create a clinically problematic situation. It should be apparent that a great deal of additional study using modern molecular and amplification methods with complex microbial communities is necessary before the parameters of natural antibiotic resistance gene transfer can be defined properly.

Other multiple resistance plasmids

Analyses of the integron-type transposons provide a good model for the way in which antibiotic resistance genes from various (unknown) sources may be incorporated into an integron by recombination events into mobile elements and hence into bacterial replicons, providing the R plasmids that we know today (Fig. 3.2) (Bissonnette and Roy, 1992). However, in bacterial pathogens a variety of transposable elements have been found that undergo different processes of recom-binational excision and insertion. It is not known what evolutionary mechanisms are implicated or whether some form of integron-related structure is present in all cases. For the type of integron found in the Tn22 family, we have plausible models, supported by in vivo and in vitro studies, to provide a modus operandi by which antibiotic resistance genes were (and are) molecularly cloned in the evolution of R plasmids. A large number of transposable elements carrying virtually all possible combinations of antibiotic...

Antibiotic producers

Covalent modification of their inhibitory biochemical products is very common in antibiotic-producing bacteria. It was the discovery of antibiotic modification as a means of self-protection in the streptomycetes that led to the proposal that antibiotic-producing microbes were the origins of the antibiotic resistance determinants found in other bacteria (Benveniste and Davies, 1973 Walker and Skorvaga, 1973). Support for this hypothesis has been provided by nucleic acid and protein sequence comparisons of aminoglycoside resistance determinants from producing organisms and clinical isolate sources (Shaw, 1984 Davies, 1992). As mentioned above, producing organisms are not the only potential source of antibiotic resistance mechanisms. The proposal that the enzymes that modify aminoglycosides evolved from such housekeeping genes as the sugar kinases and acyltrans-ferases has been made by a number of groups (Udou et al., 1989 Shaw et al, 1992 Rather et al, 1993).

Problems Associated With Biocide

Microorganisms may also gain the capacity to resist the biocide by the acquisition of gene function(s). These gene functions are mostly concerned with inactivation or modification of the biocide, efflux systems, specification of a new target, or enzymatic modification of the target. Microorganisms could also simply persist in the presence of the biocide. This phenomenon may result from mutation or temporary resistance due to gene regulatory events or phenotypic changes. It is accepted that the general resistance mechanisms producing bio-cide resistance in microorganisms are the same mechanisms found in antibiotic resistance. Excessive use of biocides may produce organisms with a non-specific mechanism(s) of cross-resistance to other biocides and, most important, to antibiotics (double resistance to biocides and to antibiotics).

And Concerns Over Resistance Development

With the ever increasing use of antimicrobials in consumer products and biocides to control biodeterio-ration, there is a recent but growing and legitimate concern over cross-resistance of biocide resistant organisms to antibiotics. It is possible that selection of microorganisms with extremely low permeability as a result of bio-cide treatment may indeed produce populations less accessible to antibiotics. Although similarities between the mechanisms of resistance development to biocides and antibiotics exist, efflux pumps, thickening of the cell wall, outer membrane alterations in gram-negative bacteria, it has not yet been established that biocide use will result in selection of an antibiotic resistance trait in the same organisms.

Distribution and diversity

Janulaitis and colleagues screened natural isolates of E. coli for sequence-specific ENases and detected activity in 25 of nearly 1000 strains tested. Another screening experiment searched for restriction activity encoded by transmissible resistance plasmids in E. coli. The plasmids were transferred to E. coli and the EOP of phage A was determined on the exconjugants. Approximately 10 of the transmissible antibiotic resistance plasmids were correlated with the restriction of A and the ENases responsible were shown to be Type II. However, plasmid-borne Type I, Type II, and Type III systems are known in E. coli. Because of the transmissible nature of many plasmids, the frequency with which R-M systems are transferred between strains could be high and their maintenance subject to a variety of selection pressures not necessarily associated with the restriction phenotype.

Ways In Which The Genome Can Change

As in the case of site-specific recombination, certain specialized short DNA sequences are involved in recombination involving DNA elements called transposons. Transposons are sequences of DNA, usually in the size range of 700-10,000 bases, which can move by recombination from one location in the genome to any of a multitude of other sites (targets). The specialized sequences that promote this recombination are usually 15-30 bases long and located at the two ends of the transposon as an inverted repeat. The DNA between the two ends can encode a number of genes whose functions are either required for the transpositional recombination (e.g. transposase, resolvase) or other functions that alter cell phenotype, such as antibiotic resistance. Transposons that do not encode any internal genes except those needed for transposition are called insertion sequences (IS). The E. coli chromosome contains several copies of five IS, and approximately 15 of the spontaneous mutations in E. coli occur...

The Risks And Consequences

Is the essence of the debate over the risk of gene escape from genetically modified organisms and the evolution of resistance to new generation antimicrobial agents. In both cases, we are interested in the formation of genotypes that produce organismal phe-notypes better avoided. Experiments that have failed to detect the exchange of genes between organisms because no recombinants were detected have suffered from two important flaws. First, the scale of the experiments and the exposure to HME invasions were both too limited to represent the fate of genes in time and on global scales. The pace and extent of antibiotic resistance evolution is a clear indication of how extremely rare events (10 72) can become certainties for populations as large as those of the HMEs and microbial cells. Second, preservation of horizontally transferred genes is the last step in the process of generating recombinant organisms. The rate limiting step in gene transmission is compromising the barriers to...

Concluding Remarks For Now And The Future

It should be apparent from the foregoing discussion of antibiotic modification that there must be a substantial pool of antibiotic resistance genes (or close relatives of these genes) in nature. Gene flux between bacterial replicons and their hosts is likely to be the rule rather than the exception, and it appears to respond quickly to environmental changes (Levy and Novick, 1986 Levy and Miller, 1989 Hughes and Datta, 1983). This gene pool is readily accessible to bacteria when they are exposed to the strong selective pressure of antibiotic usage in hospitals, for veterinary and agricultural purposes, and as growth promoters in animal and poultry husbandry. It is a life-or-death situation for microbes, and they have survived. A better knowledge The development of resistance to antimicrobial agents is inevitable, in response to the strong selective pressure and extensive use of antibiotics. Resistance may develop as the result of mutation or acquisition, or a combination of the two....

Antibiotic Susceptibility

Antibiotic resistance in Citrobacter species, as in other bacteria causing nosocomial infections, is an emerging problem. Citrobacter species without acquired antibiotic resistance are susceptible to sulfonamides, trimethoprim, aminoglycosides, chloramphenicol, tetracycline, nalidixic acid, flu

Conjugative Pilus Assembly Pathway

In gram-negative bacteria, certain pili, collectively known as conjugative pili, facilitate the interbacterial transfer of DNA. These pili allow donor and recipient bacteria to make specific and stable intercellular contacts before DNA transfer is initiated. In some cases, conjugative pili may also form the conduits for intercellular DNA transfer. Horizontal gene transfer, or conjugation, mediated by conjugative pili is inextricably associated with the spread of antibiotic resistance among bacterial pathogens. Conjugative pili are generally encoded by self-transmissible plasmids that are capable of passing a copy of their genes to a recipient bacterium. Closely related plasmids, with similar replication control systems, are unable to coexist in the same cell. This property has been termed incompatibility, and provides the primary basis for cataloging conjugal plasmids and the pili that they encode. Thus far, in E. coli alone, over 25 incompatibility groups made up of well over 100...

Genetic engineering

There has been considerable controversy regarding the potential for genetically engineered organisms to serve as effective BW agents. Recombinant DNA technology has been cited as a method for creating novel, pathogenic microorganisms. Theoretically, organisms could be developed that would possess predictable characteristics, including antibiotic resistance, altered modes of transmission, and altered pathogenic and immuno-genic capabilities. This potential for genetic engineering to significantly affect the military usefulness of BW has been contested. It has been suggested that because many genes must work together to endow an organism with pathogenic characteristics, the alteration of a few genes with recombinant DNA technology is unlikely to yield a novel pathogen that is significantly more effective or usable than conventional BW agents.

Transduction

Transduction is the exchange of bacterial genes mediated by bacteriophage, or phage. When a phage infects a cell, the phage genes direct the takeover of the host DNA and protein synthesizing machinery so that new phage particles can be made. Transducing particles are formed when plasmid DNA or fragments of host chromosomal DNA are erroneously packaged into phage particles during the replication process. Transducing particles (those carrying nonphage DNA) are included when phage are liberated from the infected cell to encounter another host and begin the next round of infection. Although there are many laboratory studies of transduction of antibiotic resistance, this mechanism has been considered less important in the dissemination of antibiotic resistance genes because phage generally have limited host ranges they can infect only members of the same or closely related species, and the size of DNA transferred does not usually exceed 50 kb. However, phage of extraordinarily broad host...

Conjugation

Conjugation is the process in which DNA is transferred during cell-to-cell contact. It has long been considered the most important mechanism for the dissemination of antibiotic resistance genes. During an epidemic of dysentery in Japan in the late 1950s increasing numbers of Shigella dysenteriae strains were isolated that were resistant to up to four antibiotics simultaneously. It soon became clear that the emergence of multiply resistant strains could not be attributed to mutation. Furthermore, both sensitive and resistant Shigella could be isolated from a single patient, and the Shigella sp. and E. coli obtained from the same patients often exhibited the same multiple resistance patterns. These finding led to the discovery of resistance transfer factors and were also an early indication of the contribution of conjugative transfer to the natural evolution of new bacterial phenotypes. In addition to plasmid-mediated conjugal transfer, another form of conjugation has been reported to...

Transformation

Natural transformation is a physiological process characteristic of many bacterial species in which the cell takes up and expresses exogenous DNA. Although natural transformation has been reported to occur only in a limited number of genera, these include many pathogenic taxa such as Haemophilus, Mycobacterium, Streptococcus, Neisseria, Pseudomonas, and Vibrio (Stewart, 1989). Initial studies suggested that natural transformation in some of these genera was limited to DNA from that particular species. For example, an 11-bb recognition sequence permits Haemophilus influenzae to take up its own DNA preferentially compared to het-erologous DNAs (Kahn and Smith, 1986). Given such specificity one could ask if natural transformation is an important mechanism in the transfer of antibiotic resistance genes. Spratt and co-workers have reported the transfer of penicillin resistance between S. pneumo-niae and N. gonorrhoeae by transformation. Further, Roberts reported that when the TetB...

Anticoagulant

Antibiotic resistance gene Gene that encodes an enzyme that degrades or excretes an antibiotic, thereby conferring resistance. Frequently found in cloning vectors like plasmids, and sometimes in natural populations of bacteria. Example bacterial ampicillin resistance is conferred by expression of the beta-lactamase gene.

Artificial Viruses

Antibiotics are chemicals produced naturally by fungi to kill their rivals bacteria. But when man began to use antibiotics, he found that, with disappointing speed, the bacteria were evolving the ability to resist the antibiotics. There were two startling things about antibiotic resistance in pathogenic bacteria. One, the genes for resistance seemed to jump from one species to another, from harmless gut bacteria to pathogens, by a form of gene transfer not

Infectious Disease

Most patients who require the expertise of these clinicians have diseases that are short-term in nature. Thus, infectious disease specialists typically serve as consultants for other physicians. In the summer of 2002, they were on the front lines of the West Nile virus outbreak in the United States. They consult on patients in the hospital for diagnostic challenges (e.g., fever of unknown origin) and for treatment regimens of specific infectious diseases (e.g., bacterial endocarditis, meningitis, cellulitis, sepsis). Many infectious disease physicians maintain longer relationships with patients suffering from chronic diseases, such as HIV AIDS and tuberculosis, who require extensive follow up. Some practice travel medicine, serving as consultants to patients preparing for international travel and to those who acquired illnesses while overseas. Other areas of expertise include infection control within health care settings, international public health, and the prevention of antibiotic...

The Bacteria Eaters

One treatment that may prove to be a solution to the problem of antibiotic resistance comes from an unlikely source viruses. It is a method developed in Russia before Fleming discovered penicillin. To modern mentality it seems bizarre, for it pits microbes against each other.

Genetics

The most extensive area of genetic research in Bacteroides is the study of antibiotic resistance and the elements involved in the transfer of resistance genes. Aside from this area of research, the genetic analysis of other virulence factors are being performed. The gene encoding the metalloprotease toxin (bft) of B. fragilis is contained on a pathogenicity island that is present only in enterotoxigenic strains. Other products involved in aerotolerance of B. fragilis, such as catalase and superoxide dismutase, have also been studied at the molecular level.

The lytic cycle

Transduction is transfer of host DNA by viruses and is normally a rare event. In generalized transduction, fragments of bacterial DNA are packaged by accident into phage heads and transferred to a new bacterium. Any host gene may be transferred and the implicated phages may be virulent or temperate. Specialized transduction is carried out by temperate phages that can integrate into host DNA (e.g., X). If the phage DNA is not properly excised, bacterial genes adjacent to the prophage site may be packaged into phage heads along with normal genes. The resulting particle has a defective genome and may be nonviable. In conversion, bacteria acquire new properties through lyso-genization by normal temperate phages. Conversion is a frequent event, affecting the whole bacterial population that has been lysogenized. The new properties are specified by phage genes and include new antigens, antibiotic resistance, colony characteristics, or toxin production (e.g., of diphtheria or botulinus...

The integron model

Studies by Hughes and Datta of plasmids they isolated from the Murray collection (Hughes and Datta, 1983 Datta and Hughes, 1983) suggest that the appearance of resistance genes is a recent event, that is, the multiresistance plasmids found in pathogens must have been created since the 1940s. What really takes place when a new antimicrobial agent is introduced and plasmid-determined resistance develops within a few years The most significant component of the process of antibiotic resistance flux in the microbial population is gene pickup, which has now been emulated in the laboratory. Largely due to the studies of Hall and co-workers (Stokes and Hall, 1989 Collis et al, 1993 Recchia and Hall, 1995), we have a good idea of the way in which transposable elements carrying multiple antibiotic resistance genes might be formed. From their studies of the organization of transposable elements, these researchers have identified a key structural constituent of one class of transposon that they...

Introduction

The development of antibiotic resistance can be viewed as a global problem in microbial genetic ecology. It is a very complex problem to contemplate, let alone solve, due to the geographic scale, the variety of environmental factors, and the enormous number and diversity of microbial participants. In addition, the situation can only be viewed retrospectively, and what has been done was uncontrolled and largely unrecorded. Simply put, since the introduction of antibiotics for the treatment of infectious diseases in the late 1940s, human and animal microbial ecology has been drastically disturbed. The response of microbes to the threat of extinction has been to find genetic and biochemical evolutionary routes that led to the development of resistance to every antimicrobial agent used. The result is a large pool of resistance determinants in the environment. The origins, evolution, and dissemination of these resistance genes is the subject of this review.