The integrity of the bacterial cytoplasmic membrane is critical in maintaining the viability of cells and their metabolic functions, particularly under stress. Bacteria actively adjust membrane fluidity through changes in lipid composition in response to variations in temperature, pressure, ion concentrations, pH, nutrient availability, and xenobiotics. Fluorescence polarization methods are valuable for measuring bacterial cytoplasmic membrane fluidity. In this review we discuss the mechanisms of bacterial membrane adaptations and present data from research using 1,6-diphenyl-1,3,5-hexatirene (DPH) as a measure of membrane fluidity and phase transitions. We illustrate the range of fluidity in viable cells, extracted membranes, and liposomes under optimal and stressed physiological conditions.

ABSTRACTPrior to the introduction ofHaemophilus influenzaetype b (Hib) conjugate vaccines, invasive Hib disease affected almost exclusively children. According to some recent studies, in the postvaccine era, adults, the elderly, and immunocompromised persons can be affected more often than children. As the production of type-specific anti-capsular polysaccharide antibodies is the major defense mechanism against Hib, individuals with defects in humoral immune responses have high susceptibility to infections caused by Hib. We hypothesized that nonvaccinated adults with chronic conditions causing immunosuppression may lack protective antibody to Hib. We assessed serum anti-Hib IgG levels and bactericidal activity in 59 patients with chronic renal failure, 30 patients with type 2 diabetes mellitus, 28 patients with chronic obstructive pulmonary disease (COPD), and 20 patients with multiple myeloma compared to 32 healthy controls of similar age. Considering antibody at >0.15 μg/ml as the protective correlate in unvaccinated individuals, we detected subprotective Hib antibody levels in 29% of chronic renal failure, 20% of diabetes, 14% of COPD, and 55% of myeloma patients compared to 3% of healthy controls. Additionally, 70% of myeloma and 58% of chronic renal failure patients did not have detectable serum bactericidal activity against Hib. Among individuals with severe diseases causing secondary immunodeficiency, patients with multiple myeloma and chronic renal failure are at an increased risk of invasive Hib disease. Considering that Hib continues to circulate in the population, this study provides a rationale for the immunization of some adult patients with secondary immunodeficiency with the pediatric Hib vaccine to achieve protective immunity.

Mine samples enriched for Thiobacillus ferrooxidans at 18, 12 and 6°C, after numerous transfers in medium 9K, yielded average generation times of 23, 44, and 103 hr, respectively. Temperature characterization of 'a natural isolate, over a range of 6 to 35°C, showed that growth occurred at the temperature extremes and that the optimum temperature for growth was 25 to 30°C. A stock culture (commercially available) chosen for comparison showed a similar optimum temperature for growth, but lower growth rates at the lower temperatures examined. Growth studies of the natural isolates in the range of 2 to 35°C revealed that growth rates decreased on either side of 25 to 30°C. This effect was most pronounced in the 2 to 12~C range. These studies revealed the psychrotrophic nature of the natural T, ferrooxidans isolates, and their better growth capabilities at lower temperatures than T. ferrooxidans ATCC 33020. These observations are discussed in the light of their importance in the uranium bioleaching process, as it is currently being used.The bacterial leaching of uranium from low-grade ores, for which it is not economically feasible to concentrate uranium by conventional means, is a notable contribution to energy production. Insoluble tetravalent uranium oxide (U02) is converted to the leachable hexavalent species (UO2S04) by ferric ions (Fe3+), according to the following equation:U0z + 2Fe3 + + 5042 -UO2SO4 + 2Fe2 +The bacterium Thiobacillus ferrooxidans generates the oxidant (Fe3+) required for this reaction by oxidizing the ferrous (Fe2+) iron which often accompanies uranium

Strain variation in the acidophile Acidithiobacillus ferrooxidans was examined as a product of membrane adaptation in response to pH stress. We tested the effects of sub and supra-optimal pH in two type strains and four strains isolated from acid mine drainage water around Sudbury, Ontario, Canada. Growth rate, membrane fluidity and phase, determined from the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene, and fatty acid profiles were compared. The effect of pH 1.5 was the most pronounced compared to the other pH values of 1.8, 3.1, and 3.5. Three different types of response to lower pH were observed, the first of which appeared to maintain cellular homeostasis more effectively. This adaptive mode included a decrease in membrane fluidity and concomitant depression of the phase transition in two distinct membrane lipid components. This was explained through the increase in saturated fatty acids (predominantly 16:0 and cyclopropane 19:0 w8c) with a concomitant decrease in 18:1 w7c fatty acid. The other strains also showed common adaptive mechanisms of specific fatty acid remodeling increasing the abundance of short-chain fatty acids. However, we suspect membrane permeability was compromised due to potential phase separation, which may interfere with energy transduction and viability at pH 1.5. We demonstrate that membrane physiology permits differentiating pH tolerance in strains of this extreme acidophile.