Inactivation of Escherichia coli by citral.
OBJECTIVE
The aim was to evaluate (i) the resistance of Escherichia coli BJ4 to citral in a buffer system as a function of citral concentration, treatment medium pH, storage time and initial inoculum size, (ii) the role of the sigma factor RpoS on citral resistance of E. coli, (iii) the role of the cell envelope damage in the mechanism of microbial inactivation by citral and (iiii) possible synergistic effects of mild heat treatment and pulsed electric fields (PEF) treatment combined with citral.


RESULTS
The initial inoculum size greatly affected the efficacy of citral against E. coli cells. Exposure to 200 microl l(-1) of citral at pH 4.0 for 24 h at 20 degrees C caused the inactivation of more than 5 log(10) cycles of cells starting at an inoculum size of 10(6) or 10(7) CFU ml(-1), whereas increasing the cell concentration to 10(9) CFU ml(-1) caused <1 log(10) cycle of inactivation. Escherichia coli showed higher resistance to citral at pH 4.0 than pH 7.0. The rpoS null mutant strain E. coli BJ4L1 was less resistant to citral than the wild-type strain. Occurrence of sublethal injury to both the cytoplasmic and outer membranes was demonstrated by adding sodium chloride or bile salts to the recovery media. The majority of sublethally injured cells by citral required energy and lipid synthesis for repair. A strongly synergistic lethal effect was shown by mild heat treatment combined with citral but the presence of citral during the application of a PEF treatment did not show any advantage.


CONCLUSIONS
This work confirms that cell envelope damage is an important event in citral inactivation of bacteria, and it describes the key factors on the inactivation of E. coli cells by citral.


CONCLUSIONS
Knowledge about the mechanism of microbial inactivation by citral helps establish successful combined preservation treatments.
Somolinos
M
M
Departamento de Producción Animal y Ciencia de los Alimentos, Facultad de Veterinaria, Miguel Servet, Zaragoza, Spain.
García
D
D
Condón
S
S
Mackey
B
B
Pagán
R
R
D000080462:Acyclic Monoterpenes; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001647:Bile Acids and Salts / Q000494:pharmacology; D002462:Cell Membrane / Q000187:drug effects; D015169:Colony Count, Microbial; D004560:Electricity; D004926:Escherichia coli / Q000187:drug effects* / Q000235:genetics; D005519:Food Preservation / Q000379:methods; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D050296:Microbial Viability; D039821:Monoterpenes / Q000494:pharmacology*; D009154:Mutation; D012808:Sigma Factor / Q000235:genetics / Q000378:metabolism*; D012965:Sodium Chloride / Q000494:pharmacology; D013997:Time Factors
D000080462:Acyclic Monoterpenes; D001426:Bacterial Proteins; D001647:Bile Acids and Salts; D039821:Monoterpenes; D012808:Sigma Factor; C069567:sigma factor KatF protein, Bacteria; D012965:Sodium Chloride; C007076:citral