Abstract:
BACKGROUND: It is generally reported that garlic (Allium sativum) harms pathogenic but not beneficial bacteria.
Although numerous studies supporting the alleged garlic effects on pathogens are available, there are limited
studies to prove this claim for beneficial bacteria. We have recently shown that garlic exhibits antibacterial activity
against probiotic bifidobacteria. The aim of the current study was to elucidate the mechanism of action of garlic
clove extract (GCE) on Bifidobacterium bifidum LMG 11041, B. longum LMG 13197 and B. lactis Bb12 using Fourier
transform infrared (FT-IR) spectroscopy and flow cytometry.
METHODS: Cultures (1 × 108 CFU ml-1) were individually incubated for 6 h at 37°C in garlic clove extract containing
allicin at a corresponding predetermined minimum bactericidal concentration for each strain. For FTIR, an aliquot
of each culture was deposited on CaF2 slide and vacuum dried. The slides were immediately viewed using a Bruker
Vertex 70 V FT-IR spectrometer equipped with a Hyperion microscope and data analyzed using OPUS software
(version 6, Bruker). Spectra were smoothed with a Savitsky-Goly function algorithim, base-line corrected and
normalized. Samples for flow cytometry were stained using the Live/Dead BacLight bacterial viability kit L7012.
Data compensation and analysis was performed using a BD FACSAria and FlowJo (version 7.6.1).
RESULTS: Fourier transform infrared spectroscopy showed changes in spectral features of lipids and fatty acids in cell
membranes, proteins, polysaccharides and nucleic acids. Spectral data as per principle component analysis (PCA)
revealed segregation of control and GCE-treated cells for all the tested bifidobacteria. Flow cytometry not only
showed increase in numbers of membrane damaged and possibly lysed cells after GCE treatment, but also
displayed diffuse light scatter patterns for GCE treated cells, which is evidence for changes to the size, granularity
and molecular content of the cells.
CONCLUSION: Garlic has multiple target sites in bifidobacteria, penetrating the cell membrane and entering the
cytoplasm, where it causes changes to carbohydrates, fatty acids, proteins and nucleic acids. These changes, for
example, modification of membrane properties, may prevent exposed bifidobacteria from colonizing the intestinal
mucosa. Loss of colonization potential would render them less efficient as probiotics.
