title: Inactivation of microorganisms by low-frequency high-power ultrasound: 1. Effect of growth phase and capsule properties of the bacteria.
abstract: The aim of this study was to determine the effects of high-intensity low-frequency (20 kHz) ultrasound treatment on the viability of bacteria suspension. More specifically, we have investigated the relationship between the deactivation efficiency and the physical (size, hydrophobicity) and biological (gram-status, growth phase) properties of the microbes. Enterobacter aerogenes, Bacillus subtilis, Staphylococcus epidermidis, S. epidermidis SK and Staphylococcus pseudintermedius were chosen for this study owing to their varying physical and biological properties. The survival ratio of the bacteria suspension was measured as a function of the ultrasound power (up to 13 W) for a constant sonication time of 20 min. Transmission electron microscopy was used to evaluate the ultrasound-induced damages to the microbes. Ultrasound treatment resulted in lethal damage to E. aerogenes and B. subtilis (up to 4.5-log reduction), whereas Staphylococcus spp. were not affected noticeably. Further, E. aerogenes suspensions were more sensitive to ultrasonication in exponential growth phase than when they were in stationary phase. The results of this study demonstrate that the main reason for bacterial resistance to ultrasonic deactivation is due to the properties of the bacterial capsule. Microbes with a thicker and "soft" capsule are highly resistant to ultrasonic deactivation process.
authors.affiliation: School of Chemical Sciences, The University of Auckland, Private Bag 92019, Auckland, New Zealand; School of Biological Sciences, The University of Auckland, Private Bag 92019, Auckland, New Zealand; Institute of Food and Agricultural Standardization, China National Institute of Standardization, Beijing 10088, China.
authors.forename: Shengpu
authors.initials: S
authors.lastname: Gao
authors.forename: Gillian D
authors.initials: GD
authors.lastname: Lewis
authors.forename: Muthupandian
authors.initials: M
authors.lastname: Ashokkumar
authors.forename: Yacine
authors.initials: Y
authors.lastname: Hemar
mesh_terms: D001419:Bacteria / Q000254:growth & development* / Q000648:ultrastructure; D002214:Capsules; D050296:Microbial Viability; D013010:Sonication
chemical_list: D002214:Capsules
keywords: Bacillus subtilis; Bacteria inactivation; Bacterial capsule; Enterobacter aerogenes; High-power ultrasound; Staphylococcus epidermidis