Physicochemical properties of BCN producers were determined to identify the most feasible BCN producer(s) in terms of growth ability and activity against SPOs. The following suitability criteria were used: growth of the strains under aerobic versus anaerobic conditions, optimal growth temperatures, optimal growth pH conditions and resistance of their BCNs to proteolytic enzymes. From this initial screening of 3000 strains, 11 strains were chosen and tested further. The BCN producers selected were: L. plantarum, L. casei, Pediococcus acidilactici (PAC.1.0), P. pentosaceus 34, P. jensennii, Lactococcus lactis strains BFE 901, 902, 903, 920 and 2072, and E. faecalis AS-48 BFE 1071. The BCN producer L. plantarum, which proved to be of particular interest, was isolated from traditional African sorghum beer. These BCN producers were subjected to further growth studies (in test tubes) under various controlled low pH ranges (simulating the citrus juice pH of 3.0 - 5.0), at 30°C, determined over a 72 h period. Consequently, 7 BCN producer strains were selected based on their growth performance and ability to produce BCNs at low pH, namely; L. plantarum, L. casei strains A and B, P. acidilactici (PAC.1.0), P. pentosaceus 34 and L. lactis strains BFE 903 and 920, which were used in further experiments. After extensive screening for the most effective BCN producers against 57 SPOs, 4 strains from the above 7 were selected, of which L. plantarum was eventually chosen for further experiments.
2.3.1 Population growth curves of BCN producers
This component of the research was conducted at BFE (Inst. für Hygiene, u. Toxikologie, Karlsruhe, now also known as the Max Rubner Institute), under the supervision of Prof. Dr Wilhelm Holtzapfel. Cell counts (during log growth phase) were conducted for each organism in each experiment in order to determine cell density and BCN activity (AU/ml), relative to cell count (colony forming units; cfu/ml).
2.3.1.1 Test tube experiments
Initial growth studies of the 11 BCN producers (listed in Section 2.3), were performed in MRS, YEL and ST-1 media until mid-logarithmic phase. Turbidity was used to estimate bacterial growth by measuring optical density (OD/ml) at 600 nm, in the pH range of 3.0 - 7.0 at 30°C (duplicate values). Optical density of BCN producer cultures was determined and analysed after 24, 48 and 72 h (blank consisted of media, adjusted to specific pHs, i.e. 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 6.5 and 7.5).
2.3.1.2 Bioscreen experiments
Growth of BCN producers was measured using a Bioscreen C automated microbiology growth curve analysis system [Labsystems, Helsinki, Finland]. This is a turbidometer, measuring OD580nm of single cell types by a vertical pathway, consisting of an analyser unit, computer and printer. The measured data were stored and further processed using BioLink Software. The OD values, relative activities and cell counts were determined at various temperatures (25, 30 and 37ºC) and pHs (3.0, 3.5 and 4.0) for L. plantarum, L. casei A and B, and P. pentosaceus 34 in MRS, and pHs 3.6, 3.8 and 4.0 for L. lactis strain BFE 903 in ST-1 broth. LA (0.1 M) was used to adjust the pH. The total volume per well in the 100 well microtitre plates was 300 µl, i.e. 270 μl medium and 30 μl bacterial culture (106
cells/ml).
2.3.2 BCN production and determination of antimicrobial activity The chosen BCN producing cultures (10 μl of 106
cells/ml) were each inoculated into 100 ml MRS broth and incubated at 30°C (except E. faecalis BFE 1071 incubated at 37°C) for 24 h. The producer cultures (total volume = 100 ml) were centrifuged (8 000 x g for 10 min), the pH of the cell-free supernatants (containing BCNs) was adjusted to 7.0 with sterile 1M NaOH (when neutralisation was required) and heat treated at 95°C for 5 min to kill any live remaining BCN producer cells which may have been retained during the centrifugation step. The antimicrobial activity was determined against selected indicator SPOs by the “spot on lawn method” (Hoover and Harlander, 1993, Van Reenen et al., 1998). This involved spotting 10 µl aliquots (undiluted) of the cell-free culture supernatant of the producer strains onto MRS soft agar plates (0.75% m/v agar), containing 106
strains [L. acidophilus, L. gallinarum, Acetobacter spp. and S. thermophilus (Table 2.2) and S. cerevisiae (Table 2.3)] or isolated juice SPOs (Table 2.4). In addition, a two-fold serial dilution (1:2 - 1:1024) of each of the supernatants of the 7 selected BCN-producing LAB strains (10 μl of 106
cells/ml) were spotted onto the plates and incubated for 24 h at 30°C. The overlay plates containing target spoilage bacteria, yeasts and moulds, were spotted with BCN (3 µl BCN producer culture or 10 µl BCN supernatant) and were incubated at the selected indicator organism temperature (i.e. 25° or 30°C) for 1 - 3 days to assess inhibitory activity of the BCNs. BCNs which exhibited inhibition zones, with a diameter of ≥2 mm, were regarded as antimicrobial (Van Reenen et al., 1998). These inhibition clearing zones were quantified in millimetres, using a refinement of the quantification system initially reported by Okkers et al. (1999) in determining the inhibitory effect of L. pentosus TV35b against Clostridium tyrobutyricum, C. sporogenes, L. sake, L. fermentum, L. curvatus, L. innocua, Propionibacterium acidipropionici and Candida albicans.
SPOs were applied with and without antibiotic, i.e. L. sakei DSM 20017 (CF) with plasmid pMG25e cloned into it, with antibiotic erythromycin (5 µg/ml) and wild type L. sakei DSM 20017 (USch) without antibiotic, in order to determine if the antibiotic would be antagonistic to the applied BCN.
The BCN supernatants (as is and neutralised by pH adjustment; also from Bioscreen experiments) of L. plantarum, L. casei A and B, P. pentosaceus 34 and L. lactis strain BFE 903 (after centrifugation at 8 000 x g for 10 min and heat shocking at 95ºC for 5 min), were all tested for efficacy against SPOs (Acetobacter spp. ATCC8303, S. thermophilus NCIMB 50083, S. cerevisiae BFE isolate, Talaromyces flavus BFE 372 and Botrytis cinerea BFE 568) to determine the BCN efficacy. BCN antimicrobial activity was expressed in arbitrary activity units per ml (AU/ml) and determined as described by Hendersen et al. (1992), where an AU is defined as the reciprocal of the highest serial two-fold dilution, displaying a zone of growth inhibition (Schillinger and Lücke, 1989; Prins et al., 2010). In determining BCN activity values (AU/ml), the BCNs were diluted as a two-fold serial dilution series (e.g. AU/ml = 100, through to AU/ml = 6 400), and then spotted in corresponding positions / zones (zone 1 = 100 AU/ml, through to zone 7 = 6 400 AU/ml) onto the plates containing the selected SPO indicator organism. Inhibition / clearing zones caused by the BCN
revealed the AU values, which were indicated by the dilution factor, i.e. the higher the dilution where zone clearing still occurred, the more potent the original BCN activity was. Specific activity values, defined as (AU/ml)/(cfu/ml) were also determined.
2.3.3 BCN activity against spoilage organisms isolated from juice
Many known SPOs (occurring in fruit juices and noted in literature) were reviewed for inclusion in this research (Tables 2.2 and 2.3). Beverage SPOs (identified by Back et al., 1999; Pontius et al., 1998; Eiroa et al., 1999; Cerny et al., 1984; Ethiraj and Suresh, 1985; Annous and Kozempel, 1998; Senser et al., 1967; Mendoza et al., 1982; Tournas, 1994; de Nijs et. al., 2000; Lavermicocca et al., 2000; Magnusson and Schnürer, 2001) were sourced from various culture collections and used to test the ability of these chosen BCN-producing organisms to inhibit growth of these SPOs.
Activities of the chosen BCN producers against the main target SPOs (listed in Tables 2.2 and 2.3) were determined. The 57 SPOs identified from the culture collections were subjected to antimicrobial activity tests against the selected BCN producers described in Section 2.3.2. Proteinase K (PK) [Serva] (5 µl of a 25 mg/ml solution) was used to verify (indicated by reduction of BCN inhibition zones) that SPO growth inhibition zones caused by the BCN producers were not due to acid / chemical effects, but rather the antimicrobial activity of the BCN producer itself. The target SPO was harvested during the log phase of growth, vortexed and inoculated (5 µl containing 106
cells/ml) onto soft agar media (0.75% m/v) and incubated as an overlay on MRS agar plates (1.5% m/v). The BCN producers (3 µl containing 106
cells/ml) were spotted onto the upper agar layer (containing the SPO) and the proteinase K solution (10 µl) spotted adjacent to the BCN producer spots. The efficacy of the selected BCN producers (as viable cell colonies on MRS agar plates), as well as BCNs (cell free supernatants) were also determined against various juice SPOs isolated from fruit juice samples [from Valor (Pty) Ltd; listed in Table 2.4] during this research.
2.3.4 Heat resistance of BCN
BCN, produced by L. plantarum, was tested for thermal resistance by heating the supernatant containing BCN to 95°C for 12.5 min, as well as storage at -80°C, followed by testing for activity against an indicator SPO (i.e. S. thermophilus).
2.4 FERMENTATION OF BACTERIOCIN PRODUCERS AND GROWTH