Assessment of the antimicrobial properties of natural selective compounds of food products

Abstract

Food products, especially those of plant origin, have long been studied for several beneficial properties, one of them being their antimicrobial properties. Several classes of compounds have been associated with antimicrobial properties, including essential oils and organic acids. This research project aims to explore the antimicrobial properties of a range of natural compounds that are found to be naturally occurring in food products. Phenolic compounds are one of the classes of chemical compounds tested for, and are most abundant in include fruits, vegetables, and spices. Sugars are the second class of natural compounds that are tested for, with honey being their most reported and noteworthy food source. The bacterium that is tested against is Staphylococcus aureus, which is a Gram-positive food borne pathogen known to be responsible for a wide variety of cases of food-borne illnesses. The health risks that this bacterium is capable of imposing go further beyond cases of food contamination, as this bacterium can also cause infections in the human body, ranging from skin infections to much more serious life-threatening complications, such as toxic shock syndrome. The specific strain of S. aureus used for this experiment is S. aureus FMCC B-135, against which so far, no experiments of this kind have been documented. A 96-well microtiter plate assay is used in order to determine the extent of bacterial growth, which functions in accordance with the Beer-Lambert law in order to measure and compare the level of absorbance of each sample in the microtiter plate wells over a time span of 24 hours. The degree of absorption of each sample are dependent on the extent of growth of the absorbing species; i.e. the bacteria. Values are given in terms of optical density. The concentrations used range from 100mg/100ml to 50mg/100ml for the phenolic compounds, and 70% to 0.00625% for the sugars. Concentrations are based on the actual amount of these compounds in food, in order for the contents of the sample to act as a representative of the content in food products. Concentrations are also based on other experimental concentrations used in experiments alike. 3 different parameters of bacterial growth were analysed for, which included fraction area (gives an indication of the total amount of bacterial growth), growth rate (the rate at which the bacteria reach their maximum population count), and the maximum optical density (the peak bacterial growth reached during the duration of the experiment). Phenolic compounds proved to not demonstrate any significant antimicrobial activity against S. aureus FMCC B-135, with the only observations of hinderance of bacterial growth being witnessed by syringic acid, which at concentrations between 80mg/100ml and 50mg/100ml are seen to slightly lower the peak bacterial growth reached. The optical density values for the fraction area are also seen to be lower than 1 in the case of both syringic acid and ferulic acid this time around, with concentrations again ranging from 80mg/100ml to 50mg/100ml. On the other hand, sugars proved to possess much stronger antimicrobial properties against S. aureus, significantly diminishing bacterial activity with respect to all the different growth parameters that are analysed for. This observation can be concluded from the low optical density values that are attained. A negative correlation is observed, as the optical density values are seen to increase as the concentration of the sugars decreases. Out of all the sugars tested for, fructose is the sugar which exhibited the strongest antimicrobial activity for all growth parameters, which optical density values decreasing to a limit lower than 0.1 at the 70% (m/v) concentration. Particularly for maltose and glucose however, it was also noticed that at the lower end of the concentrations the growth rate of S. aureus FMCC B-135 is seen to increase, with optical density values extending to ranges higher than 3. Findings from this project contribute to the existing body of knowledge regarding the beneficial properties of such compounds against such pathogenic bacteria.