Science Progress (2003), 86 (4)

Subscribers may view full papers here

Weak organic acids: a panoply of

effects on bacteria

IRVIN N. HIRSHFIELD, STEPHANIE TERZULLI AND

CONOR O’BYRNE

Weak organic acids have been used for centuries to preserve foods, but

only recently has the possible mechanism for bacterial growth inhibition

been investigated. Although the lowering of internal pH was favored as the

cause of growth inhibition, the emphasis has shifted to the anion and its

specificity. There are a number of applications of weak organic acids to

foods and in the food industry be they pre-or postharvest, However, there is

concern that the ability of foodborne pathogens to adapt to these acids may

allow longer survival in these commodities and also to better survive transit

through the gastric acid barrier of the stomach. Genomic and proteomic

approaches have been applied to the identification of genes and proteins

that may allow prokaryotes to cope with organic acid stress. These technologies

in combination with genetic approaches may provide better identification

of genes essential for survival to organic acids. These acids may

have other roles: they can induce phenotypic antibiotic resistance, and the

high concentrations of these acids in the colon may signal a relationship to

diet, colonic microflora, and human health.

Keywords: organic acids, short-chain fatty acids, food preservatives,

anions, acid adaptation, acid tolerance, habituation, mar operon, cyclopropane

fatty acids, colonic weak acids.

Bacterial outer membrane and

cell wall penetration and cell

destruction by polluting chemical

agents and physical conditions

A.D.RUSSELL

In the environment, bacteria and other microorganisms are subjected to a

variety of constantly changing chemical and physical agencies. Chemical

ones include antimicrobial compounds (both biocides and antibiotics),

pollutants, drugs, cosmetic and pharmaceutical ingredients and pesticides.

The physical agents include desiccation and drying, osmotic pressure,

hydrostatic pressure, temperature and pH changes and radiations (ultraviolet,

sunlight, ionizing). Bacteria must thus adapt to survive these inimicable

conditions. Organisms such as bacterial spores usually survive,

whereas other types of microorganisms may be much more susceptible.

Depending on the type of organism, the bacterial cell wall, outer membrane

or the spore outer layers may act as permeability barriers to the

intracellular uptake of antibiotics and biocides. Some antibacterial agents

interact with, and damage or modify, the outer components. Physical

agencies are known to damage the cytoplasmic membrane or to produce

alterations in DNA or proteins or enzymes. Nevertheless, significant damage

to the cell wall or outer membrane may also occur.

Four types of organisms are considered: cocci, mycobactria, Gramnegative

bacteria and bacterial spores. The nature of the damage inflicted

on, or in some cases prevented by, their outer cell layers is discussed for

each type of organism.

Keywords: biocides, chemical pollutants, physical processes, outer

cell damage

Bacterial responses to alkaline

stress

HIROMI SAITO AND HIROSHI KOBAYASHI*

Studies of bacterial adaptation to alkaline pH have been less extensive to

date compared with those of acidic pH. Recent development of novel methods

for global analysis of gene expression under various conditions revealed

that many genes were induced at high pH. These data led us to question

why so many genes are required for adaptation to alkaline pH. The internal

pH of bacteria growing at extremely high pH remains unclear because the

methods for measuring interior acidic ÄpH developed to date are not so

accurate, but it is generally accepted that cytoplasmic pH increases with

medium alkalization, although the increase is lower than that of the change

in medium pH. Therefore, activities of enzymes working in neutral cytoplasm

may decrease with cytoplasmic alkalization under extreme alkaline

conditions. Based on these findings, we propose in this article that genes

whose products have an optimum activity at high pH are induced under

alkaline stress to compensate for the decrease in activities of systems functioning

at neutral pH.

Keywords: bacterial adaptation, alkaline stress