Cell Wall-mediated Changes in Sensitivity of Bacillus megaterium to Chlorhexidine and 2-Phenoxyethanol, Associated with Growth Rate and Nutrient Limitation

The resistance of chemostat-grown cultures of Bacillus megaterium (asporogenous) to the bactericidal action of chlorhexidine and 2-phenoxyethanol varied with growth rate and nutrient-limitation. Phosphate-limited cultures (P-lim) showed little change in sensitivity to either drug with changes in growth rate. Magnesium-limited (Mg-lim) and carbon-limited (C-lim) cultures, however, increased in sensitivity to both agents as growth rate was increased from 0.13–0.45 h. Minimum lytic concentrations of the agents were not significantly different for protoplasts prepared from these suspensions except of Mg-lim when sensitivity to chlorhexidine increased with growth rate. Lysozyme sensitivity of the cells varied with growth rate and nutrient limitation. Results support the idea that in addition to other effects of growth rate and nutrient-limitation environmentally-induced changes in envelope structure and/or composition radically influenced penetration of these agents to their targets.     

Adaptation of Pseudomonas aeruginosa to 2,2'-methylenebis (4-chlorophenol)

A culture of Pseudomonas aeruginosa, isolated from a cooling water system, was grown in the presence of sub-inhibitory concentrations of 2,2'-methylenebis(4-chlorophenol) (MBC). It adapted to increasing concentrations from an initial minimum inhibitory concentration of 36 μg ml^-1 to the highest, 80 μg ml^-1. Resistant cultures exhibited a higher survival rate when exposed to 320 μg ml^-1 than did the original strain. Lipopolysaccharide and outer membrane protein profiles were determined by SDS PAGE. No changes were detected in lipopolysaccharide profiles. The quantity of OprP, the phosphate uptake protein in the outer membrane, decreased to a low level correlating with decreased phosphate (P_i) uptake during growth. It is proposed that OprP is the place of entry for MBC and that the cell can adapt by decreasing the level of OprP in the outer membrane.     

Lipid alterations in cell envelopes of polymyxin-resistant Pseudomonas aeruginosa isolates.

The lipid composition of cells of Pseudomonas aeruginosa strains resistant to polymyxin was compared with the lipid composition of cells of polymyxin-sensitive strains as to their content of readily extractable lipids (RELs), acid-extractable lipids, the fatty acid composition of RELs, and the contents of various phospholipids in the REL fraction. The polymyxin-resistant strains had an increased content of RELs, but a decreased phospholipid content. The REL fraction from the polymyxin-resistant strains had an increased content of unsaturated fatty acids accompanied by a decreased content of cyclopropane fatty acids as compared with the fatty acid composition of RELs from polymyxin-sensitive strains. The phosphatidylethanolamine content was greatly reduced in the polymyxin-resistant strains, whereas the content of an unidentified lipid, thought to be a neutral lipid lacking either a phosphate, free amino, or choline moiety, was greatly increased. Cell envelopes of the polymyxin-resistant strains contained reduced concentrations of Mg2+ and Ca2+ as compared with the cell envelopes of polymyxin-sensitive strains. It appears that polymyxin resistance in these strains is associated with a significant alteration in the lipid composition and divalent cation content of the cell envelope.     

Effect of R-plasmid RP1 and nutrient depletion on the gross cellular composition of Escherichia coli and its resistance to some uncoupling phenols.

The resistance of Escherichia coli batch cultures depleted of carbon (C-dep), magnesium (Mg-dep), or phosphate (P-dep) against low concentrations of 3-chlorophenol, 4-chlorophenol, or 2-phenoxyethanol varied. C-dep cultures were always significantly more sensitive than Mg-dep or P-dep cultures. The presence of R-plasmid RP1 increased the sensitivity of C-dep cultures to 3- and 4-chlorophenol, yet had little effect on those cultured depleted in magnesium or phosphate ions. Cultures with R-plasmid RP1 had increased levels of beta-polyhydroxybutyrate irrespective of the nature of the depleting nutrient. P-dep bacteria had less than one-third of the phospholipid of other cell types, this deficiency being compensated for by increases in fatty acid and neutral lipid content. The reduction in phospholipid content of P-dep cultures was entirely accounted for by decreased diphosphatidylglycerol and phosphatidylethanolamine levels in these cells.     

Effect of culture conditions on batch growth of Saccharomycopsis lipolytica on olive oil

Summary Batch cultures of Saccharomycopsis lipolytica were grown in minimal medium with olive oil as carbon source. Inocula of glucose-grown cells commenced growth with little lag at rates largely unaffected by variations in the stirring rate or oil concentration. However, growth rates declined when the medium pH was below 7.0. In all cultures, media pH declined with increasing cell concentration. Cell composition during exponential growth was 42% protein and 2% fat. Carbon-limited cells maintained this composition after oil exhaustion but during nitrogen- and oxygen-limited growth, protein content decreased and fat content increased although the protein decrease was only transient with oxygen limitation. Yield coefficients for triglyceride were near unity for all cultures. Free acid concentrations rose rapidly after inoculation. As fermentations progressed, free glycerol appeared and concentrations of di- and monoglycerides passed through maximal values although peak concentrations of di- and monoglycerides persisted for extended times in oxygen- and nitrogen-limited cultures respectively. The fraction of free glycerol consumed was greater in oxygen-limited than in carbon- or nitrogen-limited culture. The basic requirements for growth of yeasts on fatty wastes are discussed with reference to these observations.     

Changes in extracellular polysaccharide content and morphology of Microcystis aeruginosa at different specific growth rates

The cyanobacterium Microcystis mainly exists in colonies under natural conditions but as single cells in typical laboratory cultures. Understanding the mechanism by which single cells form small and large colonies can provide a deeper insight into the life history of Microcystis and the mechanisms of Microcystis bloom formation. In this paper, Microcystis aeruginosa cultured under varying light intensities and temperatures exhibited different specific growth rates. Correlations were found between the specific growth rate, extracellular polysaccharide (EPS) content, and morphology of M. aeruginosa. Under low light intensities and temperatures, M. aeruginosa formed small colonies (maximum colony size approximately 100 μm) and exhibited low specific growth rates. By contrast, standard culture conditions yielded single or paired cells with high specific growth rates. Moreover, the EPS content decreased dramatically with increasing specific growth rate. A significant positive linear relationship was observed between the EPS content per cell and colony size. High EPS content and colony formation were associated with low specific growth rates. The specific growth rate in laboratory cultures was higher than the in situ growth rate under natural conditions. This result may explain why Microcystis normally exists as single cells or (more rarely) as paired cells in axenic laboratory cultures after long-term cultivation, but forms colonies under natural conditions.     

Microbial metabolism of 2-chlorophenol, phenol and rho-cresol by Rhodococcus erythropolis M1 in co-culture with Pseudomonas fluorescens P1

Chlorophenolic waste most often contains phenol and rho-cresol along with chlorophenols. A Rhodococcus erythropolis strain M1 was isolated with the ability to degrade 2-chlorophenol, phenol and p-cresol (100 mgl(-1), each) in 18, 24 and 20 h, respectively, with negligible lag. However, Rhodococcus sp. characterized by low growth rate, pose a threat to be outgrown by bacteria occurring in natural habitats. In the present study, interaction of R. erythropolis M1 with another isolated bacteria generally encountered in activated sludge for water treatment like Pseudomonas fluorescens P1 was studied. 2-chlorophenol, phenol and p-cresol were selected as the substrates for the study. Viable cell counts showed competitive interaction between the species on 2-chlorophenol and phenol. Specific growth rate of pure culture of R. erythropolis M1 was higher than P. fluorescens P1 on 2-chlorophenol. However, in mixed culture, P. fluorescens P1 showed higher growth rate. Degradation of phenol showed higher growth rate of R. erythropolis M1 both in pure and in mixed culture form. Degradation of p-cresol had shown similar counts for both populations indicating neutral type of interaction. This observation was substantiated by detecting the growth rate, where both cultures had similar growth rate in pure and in the mixed culture form. Rate of 2-chlorophenol degradation was higher when R. erythropolis M1 was used as the pure culture as compared to the degradation rates observed with the P. fluorescens P1 or with the mixed culture. However, in case of phenol and p-cresol, degradation by the mixed culture had resulted in higher degradation rates as compared to the degradation of the substrates by both the axenic cultures.     

Degradation of 4-Chlorophenol at Low Temperature and during Extreme Temperature Fluctuations by Arthrobacter chlorophenolicus A6

Low average temperatures and temperature fluctuations in temperate soils challenge the efficacy of microbial strains used for clean up of pollutants. In this study, we investigated the cold tolerance of Arthrobacter chlorophenolicus A6, a microorganism previously shown to degrade high concentrations of 4-chlorophenol at 28°C. Luciferase activity from a luc-tagged derivative of the strain (A6L) was used to monitor the metabolic status of the population during 4-chlorophenol degradation. The A6L strain could degrade 200–300 g mL^–1 4-chlorophenol in pure cultures incubated at 5°C, although rates of degradation, growth and the metabolic status of the cells were lower at 5°C compared to 28°C. When subjected to temperature fluctuations between 5 and 28°C, A6L continued to degrade 4-chlorophenol and remained active. In soil microcosm experiments, the degradation rates were significantly faster the first week at 28°C, compared to 5°C. However, this difference was no longer seen after 7 days, and equally low 4-chlorophenol concentrations were reached after 17 days at both temperatures. During 4-chlorophenol degradation in soil, CFU and luciferase activity values remained constant at both 5 and 28°C. However, once most of the 4-chlorophenol was degraded, both values decreased by 1–1.5 logarithmic values at 28°C, whereas they remained constant at 5°C, indicating a high survival of the cells at low temperatures. Because of the ability of A. chlorophenolicus A6 to degrade high concentrations of 4-chlorophenol at 5°C, together with its tolerance to temperature fluctuations and stress conditions found in soil, this strain is a promising candidate for bioaugmentation of chlorophenol-contaminated soil in temperate climates.This revised version was published online in November 2004 with corrections to Volume 48.     

Induction characteristics of reductive dehalogenation in the ortho-halophenol-respiring bacterium,Anaeromyxobacter dehalogenans

Anaeromyxobacter dehalogenans strain 2CP-C dehalogenatesortho-substituted di- and mono-halogenated phenols and couples this activity to growth. Reductive dehalogenation activity has been reported to be inducible, however,this process has not been studied extensively. In this study, theinduction of reductive dehalogenation activity by strain 2CP-C is characterized. Constitutive 2-chlorophenoldechlorination activity occurs in non-induced fumarate-grown cells, with rates averaging 0.138 mol of Cl^- h^-1 mg of protein^-1. Once induced, these cultures dechlorinate 2-chlorophenol (2-CP) at rates as high as 116 mol of Cl^-1 h^-1 mg of protein^-1. Dechlorination of 2-CP is induced by phenol,2-chlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, 2,6-dichlorophenol,and 2-bromophenol. Of the substrates tested, 2-bromophenol shows the highestinduction potential, yielding double the 2-chlorophenol dechlorination rate when compared to other inducing substrates. No induced dechlorination is observed at concentrations less than5 M 2-CP. When fumarate cultures were diluted 100-fold, fumarate reduction rates were reduced roughly according to the dilution factor, while dechlorination rates were similar in fumarate grown cells amended with 2-CP and cells diluted 100-fold prior to the addition of chlorophenol. This indicates that the majority of the fumarate-grown cells in late log phase were not induced when exposed to inducing substrates such as 2-CP. This observation may have ramifications on the success of bioaugmentation using halorespiring bacteria, which traditionally relies on growing culturesusing more readily utilized substrates. The rapid dechlorination rate and unique induction pattern also make strain 2CP-C a promising model organism for understanding the regulationof reductive dehalogenation at the enzymatic level.     

Cometabolic degradation of 4-chlorophenol by Alcaligenes eutrophus

 Alcaligenes eutrophus was grown in batch cultures using either phenol as a sole substrate or mixtures of phenol and 4-chlorophenol. Phenol was found to be the sole source for carbon and energy while 4-chlorophenol was utilized only as a cometabolite. Maximum growth rates on phenol reached only 0.26 h^-1, significantly below the growth rates reported earlier with Pseudomonas putida. The cometabolite was found to decrease biomass yield and increase lag time before logarithmic growth occurred. Both phenol and 4-chlorophenol were found to inhibit the growth rate linearly with maximum concentrations of 1080 ppm and 69 ppm respectively, beyond which no growth occurred. The best-fit parameters are incorporated into a simple, dynamic (i.e. time-varying) model capable of predicting all the batch growth conditions presented here. It is shown that P. putida is capable of faster bioremediation when phenol is the sole carbon source or for mixed substrates with low concentrations of the cometabolite, but for high concentrations of 4-chlorophenol, A. eutrophus becomes superior because of the long lag times that occur in the Pseudomonas species. Received: 25 January 1996/Received revision: 13 March 1996/Accepted: 15 April 1996     

Effect of dilution rate on growth, productivity, cell cycle and size, and shear sensitivity of a hybridoma cell in a continuous culture

To study the effects of the growth rate of the hybridoma cell Mn12 on productivity, cell cycle, cell size, and shear sensitivity, six continuous cultures were run at dilution rate of 0.011, 0.021, 0.023, 0.030, 0.042, and 0.058 h(-1). This particular hybridoma cell appeared to be unstable in continuous culture with respect to specific productivity, as a sudden drop occurred after about 30 generations in continuous culture, accompanied by the appearance of two populations with respect to the cytoplasmic lgG content. The specific productivity increased with increasing growth rate. The shear sensitivity of the cell, as measured in a small air-lift loop reactor, increased with increasing growth rate. The mean relative cell size, as determined with a flow cytometer, increased with increasing growth rates. Furthermore, the fraction of cells in the S phase increased, and the fraction of cells in the G1/G0 phase decreased with increasing growth rates. (c) 1993 John Wiley & Sons, Inc.     

Regulation of the Expression of the Photosynthetic Apparatus of Rhodobacter capsulatus Grown in Nitrogen-Limited Chemostat Cultures

Rhodobacter capsulatus was grown in chemostat cultures under different dilution rates and with ammonium ions as the limiting nutrient. The maximal growth rate (μmax) and the Monod cell growth saturation coefficient (K_s), were calculated from batch cultures grown at different concentrations of NH₄ ⁺. The experiments in chemostat were carried out at 0.25 mM (NH₄)₂SO₄, and the dilution rates were varied between 38% and 75% of μmax. The results indicated that under continuous culture conditions the cell yield coefficient (Y) (mg dry weight × μmol consumed ammonium sulfate⁻¹) decreased with increasing dilution rate (D). On the contrary, the cell yield was constant when expressed as mg cellular protein ×μmol consumed ammonium sulfate⁻¹. This occurred as a consequence of both an increase in the consumed ammonium sulfate and a simultaneous decrease in the cell biomass production at increasing growth rates. The cells produced at higher growth rates had a higher protein content per cell. The specific content of bacteriochlorophyll (Bchl) decreased (between 3 and 4 times) with increasing growth rates measured in either cells or chromatophores. However, the absorption spectra of the cells indicated that the ratio LHI (light-harvesting complex I) to LHII (light-harvesting complex II) Bchl complexes did not change. The reaction center (RC) complex content varied in parallel with the total Bchl content, yielding a constant photosynthetic unit of 65 mol Bchl × mol RC⁻¹ at different Ds. On the other hand, the uncoupled ATPase-specific activity measured in chromatophores was usually between 30% and 40% higher at the highest growth rates reached in these experiments. Received: 22 January 1996 / Accepted: 9 March 1996     

Influence of specific growth rate and nutrient limitation upon the sensitivity of Escherichia coli towards chlorhexidine diacetate

The sensitivity of Escherichia coli to chlorhexidine has been assessed for cells grown in a chemostat at a variety of specific growth rates, under conditions of carbon, nitrogen, phosphorus and magnesium limitation. At slow rates of growth (ca 0.08/h) little difference in sensitivity was observed. As growth rate was increased, however, the sensitivity of nitrogen- and carbon-limited cells increased whilst that of magnesium- and phosphate-limited cells decreased. It was not possible to correlate the observed patterns of chlorhexidine sensitivity with any single measure of cell envelope composition (phospholipid content, lipopolysaccharide, envelope proteins, etc.). The results presented are not consistent, therefore, with any simple model for chlorhexidine binding or action and more probably reflect subtle interaction between chlorhexidine, phospholipid-lipopolysaccharide complexes and cations within the envelope.     

Influence of specific growth rate and nutrient limitation upon the sensitivity of Escherichia coli towards chlorhexidine diacetate

The sensitivity of Escherichia coli to chlorhexidine has been assessed for cells grown in a chemostat at a variety of specific growth rates, under conditions of carbon, nitrogen, phosphorus and magnesium limitation. At slow rates of growth ( ca 0.08/h) little difference in sensitivity was observed. As growth rate was increased, however, the sensitivity of nitrogen- and carbon-limited cells increased whilst that of magnesium- and phosphate-limited cells decreased. It was not possible to correlate the observed patterns of chlorhexidine sensitivity with any single measure of cell envelope composition (phospholipid content, lipopolysaccharide, envelope proteins, etc.). The results presented are not consistent, therefore, with any simple model for chlorhexidine binding or action and more probably reflect subtle interaction between chlorhexidine, phospholipid-lipopolysaccharide complexes and cations within the envelope.     

Growth and enzymological characteristics of a pink-pigmented facultative methylotrophMethylobacterium sp. MB1

Growth characteristics of batch and continuous cultures of the pink facultative methylotrophMethylobacterium sp. MB1 were determined. The response of a chemostat culture to a pulse increase of methanol concentration was studied. Malate, succinate and oxaloacetate additions to the methanol-supplemented medium decreased batch culture growth inhibition by methanol. The carotenoid content in cells grown in a chemostat decreased with increasing growth rate. The key enzyme activities of C₁-metabolism were measured in a chemostat culture at different dilution rates.     

Effect of initial cell density on hybridoma growth, metabolism, and monoclonal antibody production.

A murine hybridoma cell line (167.4G5.3) was cultivated in batch mode with varying inoculum cell densities using IMDM media of varying fetal bovine serum concentrations. It was observed that maximum cell concentrations as well as the amount of monoclonal antibody attainable in batch mode were dependent on the inoculum size. Specifically, cultures with lower inoculum size resulted in lower cell yield and lower antibody concentrations. However, in the range of 10(2) to 10(5) cells per ml, the initial cell density affected the initial growth rate by a factor of only 20%. Furthermore, specific monoclonal antibody production rates were independent of initial cell density and the serum concentration. Glutamine was the limiting nutrient for all the cultures, determining the extent of growth and the amount of antibody produced. Serum was essential for cell growth and cultures with initial cell concentrations up to 10(6) cells per ml could not grow without serum. However, when adapted, the cells could grow in a custom-made serum-free medium containing insulin, transferrin, ethanolamine, and selenium (ITES) supplements. The cells adapted to the ITES medium could grow with an initial growth rate slightly higher than in 1.25% serum and the growth rate showed an initial density dependency-inocula at 10(3) cells per ml grew 30% slower than those at 10(4) or 10(5). This difference in growth rate was decreased to 10% with the addition of conditioned ITES medium. The addition of conditioned media, however, did not improve the cell growth for serum-containing batches.     

Chlorophenol degradation coupled to sulfate reduction.

We studied chlorophenol degradation under sulfate-reducing conditions with an estuarine sediment inoculum. These cultures degraded 0.1 mM 2-, 3-, and 4-chlorophenol and 2,4-dichlorophenol within 120 to 220 days, but after refeeding with chlorophenols degradation took place in 40 days or less. Further refeeding greatly enhanced the rate of degradation. Sulfate consumption by the cultures corresponded to the stoichiometric values expected for complete oxidation of the chlorophenol to CO2. Formation of sulfide from sulfate was confirmed with a radiotracer technique. No methane was formed, verifying that sulfate reduction was the electron sink. Addition of molybdate, a specific inhibitor of sulfate reduction, inhibited chlorophenol degradation completely. These results indicate that the chlorophenols were mineralized under sulfidogenic conditions and that substrate oxidation was coupled to sulfate reduction. In acclimated cultures the three monochlorophenol isomers and 2,4-dichlorophenol were degraded at rates of 8 to 37 mumol liter-1 day-1. The relative rates of degradation were 4-chlorophenol greater than 3-chlorophenol greater than 2-chlorophenol, 2,4-dichlorophenol. Sulfidogenic cultures initiated with biomass from an anaerobic bioreactor used in treatment of pulp-bleaching effluents dechlorinated 2,4-dichlorophenol to 4-chlorophenol, which persisted, whereas 2,6-dichlorophenol was sequentially dechlorinated first to 2-chlorophenol and then to phenol.     

Degradation by and toxicity to bacteria of chlorinated phenols and benzenes,and hexachlorocyclohexane isomers

Mixed cultures degrading chlorinated benzenes, chlorinated phenols, or hexachlorocyclohexane (HCH) as the sole source of carbon and energy were obtained by enrichment from contaminated soil samples. Cultures which metabolized 3-chlorophenol (3-CP), 2,3-dichlorophenol (2,3-DCP), or 2,6-dichlorophenol (2,6-DCP) were able to utilize several other chlorinated compounds as substrates, whereas cultures enriched with 1,2,4,5-tetrachlorobenzene (1,2,4,5-TeCB), -HCH, or -HCH did not metabolize most of the other chlorinated congeners tested. Chloride release and growth rates with all four chlorinated phenols decreased with increasing initial substrate concentrations within the range of 30–250 mol liter^–1. Maximum chloride release was 3.8 mg liter^–1 corresponding to 35 mol liter^–1 trichlorophenol within 7 weeks. In contrast, the rate of metabolism of the nonphenolic compounds 1,2,4,5-TeCB, -HCH, or -HCH increased with increasing substrate concentrations. Initial concentrations of 750 mol liter^–1 -HCH or 1,2,4,5-TeCB were completely dechlorinated within 2 weeks. Because aqueous solubility and bioavailability of the chlorophenolic compounds is much higher than that of the nonphenolic compounds, it is suggested that the high bioavailability of the chlorophenolic compounds is the reason for the high toxicity of these substrates to the degrading cultures. In contrast, the low aqueous solubilities of the chlorinated benzenes and HCH-isomers caused consistently low concentrations in the medium, which were high enough to induce degradation but too low to damage the bacterial cells.Correspondence to: E. Lang     

Mechanisms of microbial movement in subsurface materials.

The biological factors important in the penetration of Escherichia coli through anaerobic, nutrient-saturated, Ottawa sand-packed cores were studied under static conditions. In cores saturated with galactose-peptone medium, motile strains of E. coli penetrated four times faster than mutants defective only in flagellar synthesis. Motile, nonchemotactic mutants penetrated the cores faster than did the chemotactic parental strain. This, plus the fact that a chemotactic galactose mutant penetrated cores saturated with peptone medium at the same rate with or without a galactose gradient, indicates that chemotaxis may not be required for bacterial penetration through unconsolidated porous media. The effect of gas production on bacterial penetration was studied by using motile and nonmotile E. coli strains together with their respective isogenic non-gas-producing mutants. No differences were observed between the penetration rates of the two motile strains through cores saturated with peptone medium with or without galactose. However, penetration of both nonmotile strains was detected only with galactose. The nonmotile, gas-producing strain penetrated cores saturated with galactose-peptone medium five to six times faster than did the nonmotile, non-gas-producing mutant, which indicates that gas production is an important mechanism for the movement of nonmotile bacteria through unconsolidated porous media. For motile strains, the penetration rate decreased with increasing galactose concentrations in the core and with decreasing inoculum sizes. Also, motile strains with the faster growth rates had faster penetration rates. These results imply that, for motile bacteria, the penetration rate is regulated by the in situ bacterial growth rate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms of microbial movement in subsurface materials

The biological factors important in the penetration of Escherichia coli through anaerobic, nutrient-saturated, Ottawa sand-packed cores were studied under static conditions. In cores saturated with galactose-peptone medium, motile strains of E. coli penetrated four times faster than mutants defective only in flagellar synthesis. Motile, nonchemotactic mutants penetrated the cores faster than did the chemotactic parental strain. This, plus the fact that a chemotactic galactose mutant penetrated cores saturated with peptone medium at the same rate with or without a galactose gradient, indicates that chemotaxis may not be required for bacterial penetration through unconsolidated porous media. The effect of gas production on bacterial penetration was studied by using motile and nonmotile E. coli strains together with their respective isogenic non-gas-producing mutants. No differences were observed between the penetration rates of the two motile strains through cores saturated with peptone medium with or without galactose. However, penetration of both nonmotile strains was detected only with galactose. The nonmotile, gas-producing strain penetrated cores saturated with galactose-peptone medium five to six times faster than did the nonmotile, non-gas-producing mutant, which indicates that gas production is an important mechanism for the movement of nonmotile bacteria through unconsolidated porous media. For motile strains, the penetration rate decreased with increasing galactose concentrations in the core and with decreasing inoculum sizes. Also, motile strains with the faster growth rates had faster penetration rates. These results imply that, for motile bacteria, the penetration rate is regulated by the in situ bacterial growth rate.(ABSTRACT TRUNCATED AT 250 WORDS)