Feng Guoping, Hew Amanda, Manoharan Ramesh, Subramanian Siva
Olam Spices and Vegetable Ingredients, Innovation and Quality, 205 East River Park Place, Suite 310, Fresno, California 93720, USA.
J Food Prot. 2017 Jul;80(7):1117-1122. doi: 10.4315/0362-028X.JFP-16-473.
Consistent deviations of the 3M Petrifilm aerobic counts (AC) from the standard pour plate aerobic plate count (APC) were observed with dehydrated onion and garlic products. A large study was designed to determine the relationship of these two methods and the root cause for the deviations. A total of 3,800 dehydrated onion and garlic samples were analyzed by both the Petrifilm AC and the standard pour plate APC method. Large spreader-like liquefied areas were observed on numerous Petrifilm plates. These liquefied areas made enumeration inaccurate. "Liquefier" microorganisms from Petrifilm plates were isolated and identified to species level by 16S rRNA and gyrB gene sequencing. Enzyme diffusion assay was performed to determine potential enzymatic degradation of guar gum, the gelling agent used in Petrifilm plates. The results indicated that the correlation between Petrifilm AC and standard APC is relatively low. Paired t test results suggested that the Petrifilm AC method produced significantly different results compared with standard APC. The discrepancies were attributable at least partly to a liquefier organism that hydrolyzed guar gum, leading to liquefaction. Liquefaction of Petrifilm plates seems to have two effects on accuracy: (i) liquefied areas may allow motile organisms to move and multiply in the liquefied area during the incubation period, yielding more than one colony from one cell and, as a result, leading to overestimation of the microbial load and (ii) the blurred areas obscure other colonies, leading to potential underestimation. The liquefier organism was identified as Bacillus amyloliquefaciens , a potent mannanase producer and heat-resistant spore former. Enzyme diffusion assay confirmed that mannanase contained in the cell-free supernatant of B. amyloliquefaciens can hydrolyze the 1,4-β-mannopyranosyl bond, the backbone of guar gum. This is the first report of the role of B. amyloliquefaciens in the liquefaction of Petrifilm plates and its negative impact on accuracy.
在脱水洋葱和大蒜制品中观察到3M Petrifilm需氧菌计数（AC）与标准倾注平板需氧菌计数（APC）存在持续偏差。为此设计了一项大型研究，以确定这两种方法之间的关系以及偏差的根本原因。总共3800份脱水洋葱和大蒜样品分别采用Petrifilm AC法和标准倾注平板APC法进行分析。在众多Petrifilm平板上观察到类似大片蔓延的液化区域。这些液化区域使得计数不准确。通过16S rRNA和gyrB基因测序，从Petrifilm平板上分离出“液化菌”微生物并鉴定到种水平。进行酶扩散试验以确定Petrifilm平板中使用的胶凝剂瓜尔胶是否存在潜在的酶促降解。结果表明，Petrifilm AC与标准APC之间的相关性相对较低。配对t检验结果表明，与标准APC相比，Petrifilm AC法产生的结果存在显著差异。这些差异至少部分归因于一种能够水解瓜尔胶导致液化的液化菌。Petrifilm平板的液化似乎对准确性有两个影响：（i）液化区域可能使活动微生物在培养期间在液化区域移动和繁殖，导致一个细胞产生多个菌落，从而导致微生物负荷估计过高；（ii）模糊区域会掩盖其他菌落，导致可能的估计过低。该液化菌被鉴定为解淀粉芽孢杆菌，它是一种高效的甘露聚糖酶生产者和耐热芽孢形成菌。酶扩散试验证实，解淀粉芽孢杆菌无细胞上清液中含有的甘露聚糖酶可以水解瓜尔胶的主链1,4-β-甘露糖苷键。这是关于解淀粉芽孢杆菌在Petrifilm平板液化中的作用及其对准确性的负面影响的首次报道。
