Quantitative determination of localized tissue oxygen concentration in vivo by two-photon excitation phosphorescence lifetime measurements.

This study describes the use of two-photon excitation phosphorescence lifetime measurements for quantitative oxygen determination in vivo. Doubling the excitation wavelength of Pd-porphyrin from visible light to the infrared allows for deeper tissue penetration and a more precise and confined selection of the excitation volume due to the nonlinear two-photon effect. By using a focused laser beam from a 1,064-nm Q-switched laser, providing 10-ns pulses of 10 mJ, albumin-bound Pd-porphyrin was effectively excited and oxygen-dependent decay of phosphorescence was observed. In vitro calibration of phosphorescence lifetime vs. oxygen tension was performed. The obtained calibration constants were kq = 356 Torr(-1) x s(-1) (quenching constant) and tau0 = 550 micros (lifetime at zero-oxygen conditions) at 37 degrees C. The phosphorescence intensity showed a squared dependency to the excitation intensity, typical for two-photon excitation. In vivo demonstration of two-photon excitation phosphorescence lifetime measurements is shown by step-wise PO2 measurements through the cortex of rat kidney. It is concluded that quantitative oxygen measurements can be made, both in vitro and in vivo, using two-photon excitation oxygen-dependent quenching of phosphorescence. The use of two-photon excitation has the potential to lead to new applications of the phosphorescence lifetime technique, e.g., noninvasive oxygen scanning in tissue at high spatial resolution. To our knowledge, this is the first report in which two-photon excitation is used in the setting of oxygen-dependent quenching of phosphorescence lifetime measurements.     

Multiple feedbacks and the prevalence of alternate stable states on coral reefs

The prevalence of alternate stable states on coral reefs has been disputed, although there is universal agreement that many reefs have experienced substantial losses of coral cover. Alternate stable states require a strong positive feedback that causes self-reinforcing runaway change when a threshold is passed. Here we use a simple model of the dynamics of corals, macroalgae and herbivores to illustrate that even weak positive feedbacks that individually cannot lead to alternate stable states can nonetheless do so if they act in concert and reinforce each other. Since the strength of feedbacks varies over time and space, our results imply that we should not reject or accept the general hypothesis that alternate stable states occur in coral reefs. Instead, it is plausible that shifts between alternate stable states can occur sporadically, or on some reefs but not others depending on local conditions. Therefore, we should aim at a better mechanistic understanding of when and why alternate stable states may occur. Our modelling results point to an urgent need to recognize, quantify, and understand feedbacks, and to reorient management interventions to focus more on the mechanisms that cause abrupt transitions between alternate states.     

Pediococci Residing on Plants

The pediococci residing on plants resemble the lactobacilli, but they differ from the streptococci in their limited distribution and low population level on plants. They are a subgroup within the genus Pediococcus which grow freely in neutral media and require neither NaCl nor CO(2). They are most readily recognized by the ability to initiate growth in liquid media, acidified to pH 5.0, which contain 1.5% sodium acetate. In stained preparations the cells occur singly and in pairs, short chains, and clusters. The occurrence of two-dimensional tetrads may be rare; this varies with the individual culture and with the culture medium. The terminal pH in 2% glucose broth varies from 3.6 to 4.3. Ability to initiate growth at 45 C, production of ammonia from arginine, dissimilation of malate, and fermentation of arabinose are confirmatory characteristics. The subgroup contains only two quite similar, but differentiable, species. P. acidilactici initiates growth at 50 C and produces catalase on heated blood medium but does not produce acid-sensitive catalase; a majority of the strains fail to initiate growth at 10 C and many fail to ferment maltose and lactose. P. pentosaceus initiates growth at 10 C but not at 50 C and produces acid-sensitive catalase; catalase production on heated blood medium is transient; a majority of the cultures ferment maltose, salicin, and trehalose. No carbohydrate serves reliably to differentiate between the species. The guanine plus cytosine ratio of P. pentosaceus deoxyribonucleic acid (DNA) was determined to be 35.1 +/- 1.2 and that of P. acidilactici DNA is 38.5 +/- 0.8.     

Characteristics of the Deoxyribonucleic Acid of Tφ3, a Bacteriophage for Bacillus stearothermophilus

It is shown that the individual strands of bacteriophage Tphi3 DNA are intact and that heat-denatured Tphi3 DNA forms a bimodal distribution in a neutral CsCl density gradient.     

Lactobacilli on Plants

The distribution, enumeration, and identification of lactobacilli on vegetable plants were studied in an area described geographically as being subtropical and moist. The lactobacilli were obtained, by means of quantitative enrichment procedures in Rogosa's SL broth, from 35.3% of all samples incubated at 32 C, and from 15.4% of the samples incubated at 45 C. Less than 10 lactobacilli/g of plant material were enumerated in 54% of all positive samples. The lactobacilli were found much less frequently and in lower numbers than were streptococci or Leuconostoc mesenteroides. The most frequently isolated lactobacillus was very simular to, but not identical with, Lactobacillus fermenti. It was aerogenic, grew well at both 15 and 45 C, fermented arabinose, lactose, and sucrose, and liberated ammonia from arginine. Of the identified species, L. plantarum, L. fermenti, and L. brevis were the most frequently isolated, whereas L. casei, L. viridescens, L. cellobiosus, L. salivarius, and L. buchneri were obtained from small numbers of samples. The widespread but sporadic distribution of lactobacilli in low numbers seems to indicate that these organisms do not normally thrive on plant surfaces. A ternary cycle, beginning with intestinal waste, followed by mechanical distribution to and among plants, and return to the host via the oral cavity, is suggested.     

Occurrence of Enterococci: Bud, Blossom, and Soil Studies

The occurrence of enterococci (group D streptococci) on buds and flowers of plants and in soils has been studied. They were recovered from 27.5% of the flowers of seven species of plants, and from 6.8% of the buds of the same plants. They were recovered from 34% of the flowers of nonagricultural plants, from 32.2% of the flowers of ten species of agricultural dicotyledonous plants, and from 10.4% of the flowers of five species of grasses and cereals. The enterococci were invariably present or invariably absent from all samples taken from very few species. They occurred in small numbers on enclosed tassels and silks of corn of 22 of 60 samples, and in greater numbers on 90% or more of these after their floral parts had emerged. Interposition of a mechanical barrier reduced the incidence of recovery from flowers. The occurrence in soil, generally at a low level of population, may be correlated with occurrence on the plant growing on the soil or with nearby enterococcal-bearing plants.It is concluded that enterococci may be regarded as temporary residents on plants, capable of limited reproduction, and that they are disseminated among plants by the action of insects and wind, and spread to the ground by these agencies, gravity, and rain.     

Spherical Lactic Acid-producing Bacteria of Southern-grown Raw and Processed Vegetables

The frequency and levels of population of the spherical lactic acid-producing bacteria were determined on raw and processed yellow summer and zucchini squash, a variety of greens, green beans, okra, southern peas, and butter and lima beans, and on fresh cucumbers and corn flowers. Six taxa occurred consistently: Leuconostoc mesenteroides, yellow-pigmented streptococci, Streptococcus faecium, Aerococcus viridans, and S. faecalis and S. faecalis var. liquefaciens. The same taxa occurred with the same order of frequency on processed, frozen vegetables, but with a marked decrease in the occurrence of S. faecalis var. liquefaciens. S. lactis, S. cremoris, S. equinus, S. bovis, and pediococci were isolated infrequently. No other member of the viridans group of the streptococci and no member of the pyogenic group was isolated. Approximately 88% of the cultures were identified. Total counts of the lactic-acid-producing bacteria rarely exceeded 10⁵ per gram of sample, and there was a reduction by 90% during the second year of study, probably because of drought. Only one bacterial species was found on 40% of the raw and 34% of the processed vegetable samples. Two or more species or taxa were present on the remainder of 153 raw and 56 processed vegetable samples. A. viridans was present on squash, greens, okra, and southern peas, and its frequency of occurrence on vegetables suggests that plants are its natural habitat.