Phage typing

Abstract: Host range describes the breadth of organisms a parasite is capable of infecting, with limits on host range stemming from parasite, host, or environmental characteristics. Parasites can adapt to overcome host or environmental limitations, while hosts can adapt to control the negative impact of parasites. We consider these adaptations as they occur among bacteriophages (phages) and their bacterial hosts, since they are significant to phage use as antibacterials (phage therapy) or to protection of industrial ferments from phage attack. Initially, we address how phage host range can (and should) be defined plus summarize claims of host ranges spanning multiple bacterial genera. Subsequently, we review bacterial mechanisms of phage resistance. These include adsorption resistance, which results in reduced interaction between phage and bacterium; what we describe as "restriction," where bacteria live but phages die; and abortive infections, where both phage and bacterium die. Adsorption resistance includes loss of phage receptor molecules on hosts as well as physical barriers hiding receptor molecules (e.g., capsules). Restriction mechanisms include phage-genome uptake blocks, superinfection immunity, restriction modification, and CRISPR, all of which function postphage adsorption but prior to terminal phage takeover of host metabolism. Standard laboratory selection methods, involving exposure of planktonic bacteria to high phage densities, tend to directly select for these prehost-takeover resistance mechanisms. Alternatively, resistance mechanisms that do not prevent bacterium death are less readily artificially selected. Contrasting especially bacteria mutation to adsorption resistance, these latter mechanisms likely are an underappreciated avenue of bacterial resistance to phage attack.

Abstract: Background Salmonella is a leading cause of food-borne illness. The emergence of antimicrobial-resistant salmonella is associated with the use of antibiotics in animals raised for food; resistant bacteria can be transmitted to humans through foods, particularly those of animal origin. We identified and characterized strains of salmonella isolated from ground meats purchased in the Washington, D.C., area. Methods Salmonella was isolated from samples of ground chicken, beef, turkey, and pork purchased at three supermarkets. The isolates were characterized by serotyping, antimicrobial-susceptibility testing, phage typing, and pulsed-field gel electrophoresis. The polymerase chain reaction and DNA sequencing were used to identify resistance integrons and extended spectrum β-lactamase genes. Results Of 200 meat samples, 41 (20 percent) contained salmonella, with a total of 13 serotypes. Eighty-four percent of the isolates were resistant to at least one antibiotic, and 53 percent were resistant to at least thre...

Abstract: The increasing threat of pathogen resistance to antibiotics requires the development of novel antimicrobial strategies. Here we present a proof of concept for a genetic strategy that aims to sensitize bacteria to antibiotics and selectively kill antibiotic-resistant bacteria. We use temperate phages to deliver a functional clustered regularly interspaced short palindromic repeats (CRISPR)–CRISPR-associated (Cas) system into the genome of antibiotic-resistant bacteria. The delivered CRISPR-Cas system destroys both antibiotic resistance-conferring plasmids and genetically modified lytic phages. This linkage between antibiotic sensitization and protection from lytic phages is a key feature of the strategy. It allows programming of lytic phages to kill only antibiotic-resistant bacteria while protecting antibiotic-sensitized bacteria. Phages designed according to this strategy may be used on hospital surfaces and hand sanitizers to facilitate replacement of antibiotic-resistant pathogens with sensitive ones.

Abstract: Listeria monocytogenes is an important food-borne pathogen and is widely tested for in food, environmental and clinical samples. Identification traditionally involved culture methods based on selective enrichment and plating followed by the characterization of Listeria spp. based on colony morphology, sugar fermentation and haemolytic properties. These methods are the gold standard; but they are lengthy and may not be suitable for testing of foods with short shelf lives. As a result more rapid tests were developed based on antibodies (ELISA) or molecular techniques (PCR or DNA hybridization). While these tests possess equal sensitivity, they are rapid and allow testing to be completed within 48 h. More recently, molecular methods were developed that target RNA rather than DNA, such as RT-PCR, real time PCR or nucleic acid based sequence amplification (NASBA). These tests not only provide a measure of cell viability but they can also be used for quantitative analysis. In addition, a variety of tests are available for sub-species characterization, which are particularly useful in epidemiological investigations. Early typing methods differentiated isolates based on phenotypic markers, such as multilocus enzyme electrophoresis, phage typing and serotyping. These phenotypic typing methods are being replaced by molecular tests, which reflect genetic relationships between isolates and are more accurate. These new methods are currently mainly used in research but their considerable potential for routine testing in the future cannot be overlooked.