A critical examination of the presumptive coliform test in milk

The Presumptive Coliform test is used as an indicator of possible faecal contamination of water supplies. In the attempts to provide a bacteriological standard for milk of good keeping quality which is safe for human consumption, the same test has been applied to milk. Thus, milks of high coliform content tend to be regarded by Department of Health authorities as "not up to standard" and not suitable for marketing. This view antagonises the farmer or producer who suffers financially if his milk is withheld from the market. His immediate reaction is to condemn the coliform test as valueless because it is not a true indicator of the standard of milk. The controversy arises because, for water samples, the presumptive coliform test is a good indicator of faecal contamination but has quite a different significance in milk. A positive test with milk may indeed indicate faecal contamination but there are many other possible explanations. Above all coliforms quickly die in water and their presence therefore shows recent faecal contamination; whereas in milk the organisms not only do not die but grow abundantly. Opinion differs greatly on the validity of the test, as can be seen from the fact that in England the test was abolished in 1949 whereas it is still retained in Scotland, In America it is retained for milk of "Certified" grade. The chief aim in milk production should be to provide the consumer with milk of good keeping quality and free from any pathogenic bacteria. How does the presumptive coliform test stand in this matter? In Scotland there are three main grades of milk namely, "Certified", Tuberculin Tested (T.T.) and Pasteurised milk, and / and each has its own coliform-test standard, "Certified" milk comes from tuberculin-tested herds, is produced under very clean conditions, and is bottled on the farm. It is regarded by many people as the "best" milk. T.T, milk comes from tuberculin-tested herds and may or may not be pasteurised. In Scotland 99.8% of herds are T.T. (Chalmers and Sampson, 1958). Pasteurised milk has undergone heat treatment to destroy any pathogenic bacteria which may be present. Undoubtedly it will be the safest milk, but many people dislike its 'flat' taste and are under the impression that it loses some important fraction of its nutritive value after heat treatment. With the above points in view, it was decided to examine milks of different grades, at different seasons, as purchased by the consumer i.e. milk delivered in bottles after it had passed through all stages of production. This was decided upon because most routine testing of milk is done before the milk reaches the consumer, usually on individual samples from farms or on bulk samples at creameries. It seemed of interest to discover whether the bacteriology of the milk would shed some light on the possible status of the test by the time that the milk had actually reached its destination. Presumptive coliform tests were carried out, and positive samples were further examined to see whether coliform organisms were indeed responsible for the positive tests. Samples were also examined by other tests and some interesting points arose. One of the most surprising was the high incidence of Staphylococcus aureus in raw milk, and the revealing fact that on bacteriological grounds "Certified" milk is far from being a "good" milk compared with Pasteurised milk. The following work is therefore directed mainly towards findings on the examination of milks by the presumptive coliform test. It also includes, however, a number of relevant observations on the significant bacterial flora of consumer milk

Towards Predictive Computational Models of Oncolytic Virus Therapy: Basis for Experimental Validation and Model Selection

Oncolytic viruses are viruses that specifically infect cancer cells and kill them, while leaving healthy cells largely intact. Their ability to spread through the tumor makes them an attractive therapy approach. While promising results have been observed in clinical trials, solid success remains elusive since we lack understanding of the basic principles that govern the dynamical interactions between the virus and the cancer. In this respect, computational models can help experimental research at optimizing treatment regimes. Although preliminary mathematical work has been performed, this suffers from the fact that individual models are largely arbitrary and based on biologically uncertain assumptions. Here, we present a general framework to study the dynamics of oncolytic viruses that is independent of uncertain and arbitrary mathematical formulations. We find two categories of dynamics, depending on the assumptions about spatial constraints that govern that spread of the virus from cell to cell. If infected cells are mixed among uninfected cells, there exists a viral replication rate threshold beyond which tumor control is the only outcome. On the other hand, if infected cells are clustered together (e.g. in a solid tumor), then we observe more complicated dynamics in which the outcome of therapy might go either way, depending on the initial number of cells and viruses. We fit our models to previously published experimental data and discuss aspects of model validation, selection, and experimental design. This framework can be used as a basis for model selection and validation in the context of future, more detailed experimental studies. It can further serve as the basis for future, more complex models that take into account other clinically relevant factors such as immune responses

Evolutionary Epidemiology of Drug-Resistance in Space

The spread of drug-resistant parasites erodes the efficacy of therapeutic treatments against many infectious diseases and is a major threat of the 21st century. The evolution of drug-resistance depends, among other things, on how the treatments are administered at the population level. “Resistance management” consists of finding optimal treatment strategies that both reduce the consequence of an infection at the individual host level, and limit the spread of drug-resistance in the pathogen population. Several studies have focused on the effect of mixing different treatments, or of alternating them in time. Here, we analyze another strategy, where the use of the drug varies spatially: there are places where no one receives any treatment. We find that such a spatial heterogeneity can totally prevent the rise of drug-resistance, provided that the size of treated patches is below a critical threshold. The range of parasite dispersal, the relative costs and benefits of being drug-resistant compared to being drug-sensitive, and the duration of an infection with drug-resistant parasites are the main factors determining the value of this threshold. Our analysis thus provides some general guidance regarding the optimal spatial use of drugs to prevent or limit the evolution of drug-resistance

Association of insularity and body condition to cloacal bacteria prevalence in a small shorebird

Do islands harbour less diverse disease communities than mainland? The island biogeography theory predicts more diverse communities on mainland than on islands due to more niches, more diverse habitats and availability of greater range of hosts. We compared bacteria prevalences ofCampylobacter,ChlamydiaandSalmonellain cloacal samples of a small shorebird, the Kentish plover (Charadrius alexandrinus) between two island populations of Macaronesia and two mainland locations in the Iberian Peninsula. Bacteria were found in all populations but, contrary to the expectations, prevalences did not differ between islands and mainland. Females had higher prevalences than males forSalmonellaand when three bacteria genera were pooled together. Bacteria infection was unrelated to bird's body condition but females from mainland were heavier than males and birds from mainland were heavier than those from islands. Abiotic variables consistent throughout breeding sites, like high salinity that is known to inhibit bacteria growth, could explain the lack of differences in the bacteria prevalence between areas. We argue about the possible drivers and implications of sex differences in bacteria prevalence in Kentish plovers

Determinants of Currency Risk Premiums

The risk premium is a function of both the interest rate differential and the gap between the current exchange rate and its long-run equilibrium in a model of the foreign exchange market with both non-speculating traders and rational speculators. If the speculators have an alternative to specializing in exchange-rate speculation, then there should be no presumption that uncovered interest rate parity will hold even approximately with a long-run equilibrium number of speculators. Furthermore, when other traders respond to interest-rate differentials, the model can give rise to a negative relationship between the interest-rate differential and the subsequent change in the exchange rate, a phenomenon that is often evident in foreign exchange markets