In situ and experimental evidence for effects of elevated pH on protistan and metazoan grazers

Plankton succession was studied in a hyper-eutrophic stratified estuary, Mariager Fjord, Denmark. Above the pycnocline (15 m) pH increased from 8.5 to 9.2 and the oxygen increased to super saturation after 5 d of sunny weather due to high primary production. The protistan grazers were dominated by heterotrophic dinoflagellates and mixotrophic and heterotrophic ciliates. Metazooplankton was dominated by meroplankton, rotifers and the copepod, Acartia tonsa, all with a relatively low biomass. Cirriped nauplii occupied the upper strata while polychaete larvae populated the whole water column. Bivalve larvae occurred occasionally above the pycnocline even at very high pH. In pH challenge experiments, the mixotrophic ciliate Mesodinium rubrum was the least pH tolerant species, followed by Strombidium spp., which did not cope well with seawater pH > 8.5. Some heterotrophic dinoflagellates were more tolerant with net growth at pH > 9. The predominant rotifer Synchaeta sp. tolerated up to pH 9.5 and the copepod survived pH 10 but stopped producing eggs at pH 9.5 with unaffected egg hatching success. The polychaete and cirriped larvae tolerated pH 9.5, but bivalve larvae showed decreased survival already at pH 8.5. In situ distribution patterns and pH challenge experiments suggest that pH indeed contribute to structuring zooplankton distribution.

Stationary-Phase Acid Resistance and Injury of Recent Bovine Escherichia coli O157 and non-O157 Biotype I Escherichia coli Isolates†

Stationary-phase acid resistance and the induction of acid resistance were assessed for recent bovine carcass isolates of Escherichia coli, including 39 serotype O157 strains and 20 non-O157 strains. When grown to stationary phase in the absence of glucose and without prior acid exposure, there was a range of responses to a pH challenge of 6 h at pH 2.5. However, populations of 53 of the 59 E. coli isolates examined were reduced by less than 2.00 log CFU/ml, and populations of 24 of these isolates were reduced by less than 1.00 log CFU/ml. In contrast, there was little variation in population reductions when the E. coli were grown with glucose and preadapted to acidic conditions. With few exceptions, acid adaptation improved survival to the acid challenge, with 57 of the 59 isolates exhibiting a log reduction of less than 0.50. Differences in acid resistance or the ability to adapt to acidic conditions between E. coli O157:H7 and non-O157 commensal E. coli were not observed. However, we did find that the E. coli O157 were disposed to greater acid injury after the low pH challenge than the non-O157 E. coli, both for cells that were and were not adapted to acidic conditions before the challenge. The enhancement of low pH survival after acid adaptation that was seen among these recent natural isolates of E. coli O157 further supports the idea that the previous environment of this pathogen should be a consideration when designing microbial safety strategies for foods preserved by low pH and acid.