Infrared and Raman spectroscopic studies of the antimicrobial effects of garlic concentrates and diallyl constituents on foodborne pathogens

Abstract

The antimicrobial effects of garlic (Allium sativum) extract (25, 50, 75, 100, and 200 μL/ml) and diallyl sulfide (5, 10, and 20 μM) on Listeria monocytogenes and Escherichia coli O157:H7 cultivated in tryptic soy broth at 4, 22, and 35 °C for up to 7 days were investigated. L. monocytogenes was more resistant to garlic extract and diallyl compounds treatment than E. coli O157:H7. Fourier transform infrared (FT-IR) spectroscopy indicated that diallyl constituents contributed more to the antimicrobial effect than phenolic compounds. This effect was verified by Raman spectroscopy and Raman mapping on single bacteria. Scanning electron microscope (SEM) and transmission electron microscope (TEM) showed cell membrane damage consistent with spectroscopic observation. The degree of bacterial cell injury could be quantified using chemometric methods.

Figure 1 (a)

Different raw vibrational spectroscopic spectra of E. coli O157:H7 (1) IR transmission spectrum (2) micro-Raman spectrum with an excitation wavelength of 532 nm.

Figure 1 (b)

Different raw vibrational spectroscopic spectra of L. monocytogenes (1) IR transmission spectrum (2) micro-Raman spectrum with an excitation wavelength of 532 nm.

Figure 2

Raman mapping experiment of E. coli O157:H7 (a-c) and L. monocytogenes (d-e) cells at selective wavenumbers (a) 1208-1219 cm⁻¹ (b) 1195-1520 cm⁻¹ (c) 1525-1788 cm⁻¹ (d) 1206-1539 cm⁻¹ (e) 1535-1761 cm⁻¹.

Figure 3 (a)

Second derivative transformation of FT-IR spectral features of E. coli O157:H7 treated with 10 μM diallyl sulfide at 22°C for different time intervals (black: 0 day, purple: 1 day, green: 3 day, blue: 5 day, red: 7 day).

Figure 3 (b)

Second derivative transformation of FT-IR spectral features of E. coli O157:H7 treated with 25 μl/ml garlic concentrate at 22°C for different time intervals (black: 0 d, purple: 1 d, green: 3 d, blue: 5 d, red: 7 d). The red columns show differences in spectral features treated between garlic concentrate and organosulfur compounds.

Figure 3 (c)

Second derivative transformation of FT-IR spectral features of L. monocytogenes treated with 10 μM diallyl sulfide at 22°C for different time intervals (black: 0 d, purple: 1 d, green: 3 d, blue: 5 d, red: 7 d).

Figure 3 (d)

Second derivative transformation of FT-IR spectral features of L. monocytogenes treated with 25 μl/ml garlic concentrate at 22°C for different time intervals (black: 0 d, purple: 1 d, green: 3 d, blue: 5 d, red: 7 d). The red columns show differences in spectral features treated between garlic concentrate and organosulfur compounds.

Figure 4 (a)

Second derivative transformation of Raman spectral features of E. coli O157:H7 treated with 10 μM diallyl sulfide at 22°C for different time intervals (blue: treated for 5 days; black: control).

Figure 4 (b)

Second derivative transformation of Raman spectral features of L. monocytogenes treated with 10 μM diallyl sulfide at 22°C for different time intervals (blue: treated for 5 days; black: control).

Figure 5

Transmission electron microscope images of Escherichia coli O157:H7 without (a) and with (b) the treatment of organosulfur compounds derived from garlic (Allium sativum) and Listeria monocytogenes without (c) and with (d) the treatment of organosulfur compounds derived from garlic (Allium sativum).

Figure 6

Representative partial least squares regression for actual bacteiral survivial counts prediction in sterilized broth containing 10 μM diallyl sulfide for different time intervals at room temperature (22°C) for L. monocytogenes (a) and E. coli O157:H7 (b) using Raman spectra and L. monocytogenes (c) and E. coli O157:H7 (d) using infrared spectra.