Quantitative determination of bacterial replication in vivo

Effect of the population heterogeneity on growth behavior and its estimation

Different types of the Logistic model are constructed based on a simple assumption that the microbial populations are all composed of homogeneous members and consequently, the condition of design for the initial value of these models has to be rather limited in the case of N(t(0))=N(0). Therefore, these models cannot distinguish the dynamic behavior of the populations possessing the same N(0) from heterogeneous phases. In fact, only a certain ratio of the cells in a population is dividing at any moment during growth progress, termed as theta, and thus, dN/dt not only depends on N, but also on theta. So theta is a necessary element for the condition design of the initial value. Unfortunately, this idea has long been neglected in widely used growth models. However, combining together the two factors (N(0) and theta) into the initial value often leads to the complexity in the mathematical solution. This difficulty can be overcome by using instantaneous rates (V(inst)) to express growth progress. Previous studies in our laboratory suggested that the V(inst) curve of the bacterial populations all showed a Guassian function shape and thus, the different growth phases can be reasonably distinguished. In the present study, the Gaussian distribution function was transformed approximately into an analytical form (Y(i)=alpha(e)[-0.5(x(i)-x(0)/b)(2)]) that can be conveniently used to evaluate the growth parameters and in this way the intrinsic growth behavior of a bacterial species growing in heterogeneous phases can be estimated. In addition, a new method has been proposed, in this case, the lag period and the double time for a bacterial population can also be reasonably evaluated. This approach proposed could thus be expected to reveal important insight of bacterial population growth. Some aspects in modeling population growth are also discussed.

Antimicrobial drug delivery in food animals and microbial food safety concerns: an overview of in vitro and in vivo factors potentially affecting the animal gut microflora

This review provides an overview of considerations particular to the delivery of antimicrobial agents to food animals. Antimicrobial drugs are used in food animals for a variety of purposes. These drugs may have therapeutic effects against disease agents, or may cause changes in the structure and/or function of systems within the target animal. Routes of administration, quantity, duration, and potency of an antimicrobial drug are all important factors affecting their action(s) and success. Not only might targeted pathogens be affected, but also bacteria residing in (or on) the treated food animals, especially in the intestines (gastrointestinal tract microflora). Resistance to antimicrobial agents can occur through a number of mechanisms. The extent to which resistance develops is greatly affected by the amount of drug [or its metabolite(s)] a bacterium is exposed to, the duration of exposure, and the interaction between an individual antimicrobial agent and a particular bacterium. The impact of antimicrobial agents on the emergence of resistance in vitro and in vivo may not readily correlate.

Questions about the behaviour of bacterial pathogens in vivo

Bacterial pathogens cause disease in man and animals. They have unique biological properties, which enable them to colonize mucous surfaces, penetrate them, grow in the environment of the host, inhibit or avoid host defences and damage the host. The bacterial products responsible for these five biological requirements are the determinants of pathogenicity (virulence determinants). Current knowledge comes from studies in vitro, but now interest is increasing in how bacteria behave and produce virulence determinants within the infected host. There are three aspects to elucidate: bacterial activities, the host factors that affect them and the metabolic interactions between the two. The first is relatively easy to accomplish and, recently, new methods for doing this have been devised. The second is not easy because of the complexity of the environment in vivo and its ever-changing face. Nevertheless, some information can be gained from the literature and by new methodology. The third aspect is very difficult to study effectively unless some events in vivo can be simulated in vitro. The objectives of the Discussion Meeting were to describe the new methods and to show how they, and conventional studies, are revealing the activities of bacterial pathogens in vivo. This paper sets the scene by raising some questions and suggesting, with examples, how they might be answered. Bacterial growth in vivo is the primary requirement for pathogenicity. Without growth, determinants of the other four requirements are not formed. Results from the new methods are underlining this point. The important questions are as follows. What is the pattern of a developing infection and the growth rates and population sizes of the bacteria at different stages? What nutrients are present in vivo and how do they change as infection progresses and relate to growth rates and population sizes? How are these nutrients metabolized and by what bacterial mechanisms? Which bacterial processes handle nutrient deficiencies and antagonistic conditions that may arise? Conventional and new methods can answer the first question and part of the second; examples are described. The difficulties of trying to answer the last two are discussed. Turning to production in vivo of determinants of mucosal colonization, penetration, interference with host defence and damage to the host, here are the crucial questions. Are putative determinants, which have been recognized by studies in vitro, produced in vivo and are they relevant to virulence? Can hitherto unknown virulence determinants be recognized by examining bacteria grown in vivo? Does the complement of virulence determinants change as infection proceeds? Are regulatory processes recognized in vitro, such as ToxR/ToxS, PhoP/PhoQ, quorum sensing and type III secretion, operative in vivo? What environmental factors affect virulence determinant production in vivo and by what metabolic processes? Examples indicate that the answers to the first four questions are 'yes' in most but not all cases. Attempts to answer the last, and most difficult, question are also described. Finally, sialylation of the lipopolysaccharide of gonococci in vivo by host-derived cytidine 5'-mono-phospho-N-acetyl neuraminic acid, and the effect of host lactate are described. This investigation revealed a new bacterial component important in pathogenicity, the host factors responsible for its production and the metabolism involved.

The revival of interest in mechanisms of bacterial pathogenicity

1. After a long barren period, the study of bacterial pathogenicity is now one of the most popular subjects in microbiology. This is because bacterial diseases remain a major problem in public health despite the advent of antibiotics, and the subject is a fertile field for the application of genetics and molecular biology. 2. Pathogenicity is a multifactorial property. The biological requirements are abilities to: infect mucous surfaces; enter the host through those surfaces; multiply in the environment of the host; interfere with host defences; and damage the host. Each requirement has many facets all of which can be accomplished by a variety of processes. 3. The molecular determinants of the five requirements for pathogenicity can be identified and the relation between their structure and function obtained by a seven step procedure. Genetic manipulation and observations on organisms grown in vivo play major roles in this procedure. Other vital aspects are the availability of good animal models and the design of biological tests for virulence determinants in vitro that are pertinent to the situation in vivo. 4. A survey of the state of studies on bacterial pathogenicity has highlighted some areas of immense erudition and exposed others that need more attention in the future. Research is often at the highest level of molecular biology for: adherence to and entry of epithelial cells; interference with humoral and phagocytic defences; toxins; and direct induction of cytokines and inflammation. The major gaps are: the determinants of competition with commensals on mucous surfaces; spread into deeper tissues; the host supplied nutrients and metabolism underlying growth rate in vivo; the determinants of interference with the immune response in important chronic diseases and carrier states; the determinants of immunopathological reactions that cause damage in chronic disease; and the determinants of change from carrier to invasive state. Areas that are receiving some attention but are worthy of more are: moving through mucus to gain access to mucous surfaces; opportunistic infections; the determinants of mixed infections; and the determinants of host and tissue susceptibility to infection. 5. Current interest in the regulation of production of virulence determinants and the influence on it of environmental factors has raised speculation on the role these factors play in vivo. However, it has not yet provided much information on the host factors specifically involved in particular bacterial infections. The individualistic concept of community, as a relative latecomer to discussions of animal community, is sometimes misconstrued as holding that communities are random assemblages of organisms without biotic interactions among species. Nevertheless, it has increasingly been accepted as supported by studies of diverse taxa and habitats. However, many other ecologists continue to argue for integrated, biotically controlled and evolved communities.(ABSTRACT TRUNCATED AT 400 WORDS)

Temperature-sensitive mutants of bacterial pathogens: isolation and use to determine host clearance and in vivo replication rates

Temperature-sensitive mutants of bacterial pathogens are relatively easy to obtain and characterize. We have used ts mutants of a number of bacterial pathogens: E. coli and Listeria monocytogenes to determine quantitatively the bacterial activities in vitro of macrophages and polymorphonuclear leukocytes; E. coli and P. cepacia to study clearance of the bacteria in vivo; and Salmonella enteritidis, Listeria monocytogenes, E. coli, P. aeruginosa, and P. cepacia to determine quantitatively the replication rates in the spleens, lungs, and peritoneal cavities of mice, and the lungs of chickens.

Differential growth characteristics and immunogenicity of tight and coasting temperature-sensitive mutants of Pseudomonas aeruginosa

Genetically attenuated vaccines capable of limited replication in the vaccinate may elicit stronger, longer-lasting immunity than that induced by component, killed whole-cell, or nonreplicating live vaccines. We have isolated and partially characterized temperature-sensitive Pseudomonas aeruginosa mutants of two different phenotypes: a tight mutant, which ceases all growth immediately after its transfer to 36 degrees C, and a coaster, which continues to replicate for five generations at 36 degrees C. The growth profiles of the two temperature-sensitive phenotypes were compared both in vitro and in vivo; maintenance of the coasting phenotype in vivo was confirmed. The immunogenicity of the two phenotypes was compared in two models. In model 1, ICR mice were immunized intraperitoneally (i.p.) with graded doses of either mutant and challenged 3 weeks later i.p. with lethal doses of the wild-type strain. In model 2, DBA/2J mice were immunized intranasally with either mutant and subsequently challenged with an aerosolized inoculum of the wild-type strain, and lung clearance was measured over 4 h. In both models, the coaster demonstrated slightly higher immunogenic potential and, in addition, induced significantly higher levels of immunotype-specific serum immunoglobulin G after i.p. immunization.

Comparison of in vivo and in vitro multiplication rates of Haemophilus influenzae type b

A model of invasive Haemophilus influenzae type b disease in rats was used to compare the net in vivo and in vitro multiplication rates of this bacterium. In both the blood of rats and in vitro cultures (fresh rat blood or enriched broth) there was an exponential increase in the number of CFU per milliliter with calculated net mean generation times as follows: sham-operated rats, 82 +/- 39 min (n = 5); asplenic rats, 34 +/- 5 min (n = 13); broth, 26 +/- 4 min (n = 12); rat blood, 24 +/- 1 min (n = 14). Thus, in vivo multiplication of H. influenzae type b can be extremely efficient.

Expression of the innate resistance gene Ity in mouse Kupffer cells infected with Salmonella typhimurium in vitro

Early innate resistance to salmonellae in mice is controlled by the chromosome 1 gene Ity which regulates the in vivo net growth rate of bacteria in the RES by an unknown mechanism. It similarly controls innate resistance to Leishmania donovani, BCG and Mycobacterium lepraemurium. Murine Kupffer cell cultures were infected with virulent Salmonella typhimurium and followed for 12 h. Multiresistant organisms were used so that the antibiotics in the medium could not interfere with the results; extracellular bacteria were removed by repeated washes. Monolayers from resistant Ityr (C3H/He, CBA, A/J) and Ityr/s (B10 x A/J)F1 mice resisted infection with S. typhimurium C5 better than those from susceptible Itys (DBA/1, BALB/c, B10.M) mice, which were progressively lost from the culture plates at a faster rate than resistant monolayers. Organisms were clearly visible inside large vacuoles in the macrophages. The results confirm and amplify results of others on salmonella-infected peritoneal and splenic macrophages, and support the view that the Ity gene is expressed as a function of macrophages.

Replication rate of Pseudomonas aeruginosa in the murine lung

Using a method recently developed at our laboratory, we determined the initial rate of Pseudomonas aeruginosa replication in the lung under different experimental conditions. Mice were exposed to aerosols containing mixtures of a temperature-sensitive (ts) mutant of P. aeruginosa and its parental wild type (wt). The changes in the ratio of ts:wt were determined by quantitatively culturing homogenates of lungs from animals sacrificed over different time periods. The doubling time (DT) was calculated as the reciprocal of the slope of the linear portion of the curve generated by plotting n = (log [r0/rt])/log 2 against time where r is the ratio of ts:wt at a given time. The DTs measured in both outbred ICR mice and F1 hybrids (DBA/2J X B10.D2/nSnJ) were 32 and 30 min, respectively. These DTs were higher than that determined in the peritoneal cavities of ICR mice (20 min). The DT in the lungs of ICR mice rendered granulocytopenic by treatment with cyclophosphamide was 16 min. Experiments performed with inocula of different sizes showed that DTs tended to be higher in animals aerosolized with low doses of the ts-wt mixture.