Me And Lactic Acid Bacteria

A few years back, the products labelled “probiotic” have become popular due to their ability in stimulating the growth of beneficial flora in the gut. Such products are Yakult, Vitagen and probiotics supplement pill. Then, I realize that bacteria in probiotic products are from a group known as Lactic Acid Bacteria (LAB). LAB can be found in food and feed fermentation and most of them are beneficial microorganism. (Sampo Lahtinen et.al. , 2012) Foods such as tempeh, kimchi, miso and cheese are produced with the help of LAB. Then, a question appeared in my mind. Is there any pathogenic LAB? After reviewing a few research paper and book, genera such as Streptococcus, Lactococcus, Enterococcus, Carnobacterium contains species that are pathogenic to human or animal. During the process, I found an interesting pathogenic LAB which is Streptococcus mutans (S. mutans). This pathogen has caused various disease in different part of our body. I wonder how it cause disease and thus I choose ‘Streptococcus mutans’ as the topic discussed in this assignment, “Microbiology and Me”.

S. mutans is a cocci shaped Gram-positive, facultative anaerobe and catalase-negative bacteria with cell wall containing diamino acid lysine. It is a non-endospore-forming and have a low G+C content (≤55% mol) (B.J.B.Wood, 2014). Unlike Lactobacillus sp. which are also LAB, S. mutans is an obligate human pathogen compared to Lactobacillus sp. that have free-living lifestyle. Therefore, S. mutans has developed the mechanisms that copes with fluctuation in pH, oxygen tension and carbohydrate because of its obligate host-associated lifestyle. (J.A. Lemos et. al, 2013) Due to its relatively small genome and host-associated lifestyle, S. mutans has mechanisms of gene regulation and metabolic pathways that overlap with other related Gram-positive pathogens which have compact genomes, low GC content and host-associated lifestyle. The genome of S. mutans can be easily manipulated and relatively safe to manipulate in the laboratory setting. Thus, S. mutans act as a suitable model organism of pathogenic Gram-positive bacteria. (J.A. Lemos et. al, 2013) Moreover, S. mutans can be easily found in our oral cavity.

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S. mutans is an acid-producing and acid-tolerant species as it assimilates carbohydrate by glycolysis and produces lactic acid into their surroundings. Lactic acid is produced when S. mutans expose to environment that has excess carbohydrate source and less availability in oxygen. The production of lactic acid acidifies the environment and allow S. mutans to outcompete other acid-sensitive microorganisms in the oral cavity. (J. Abranches et.al , 2018) Next, S. mutans have few serotype which is serotype c, e, f, k that contribute to different illness such as dental caries, bacteremia, and infective endocarditis. Serotype determination is based on the sugar composition of serotype-specific polysaccharide on S. mutans. Serotype c, e, f have surface antigen that contain glucose and rhamnose however, serotype k, novel serotype has only rhamnose in their surface antigen. (Kazuhiko, N. & Takashi, O., 2009).

Young child tends to have dental caries or tooth decay due to the lactic acid produced by S. mutans. Dental caries start with the formation of dental plaque, a biofilm form by opportunistic and commensal microorganism in oral cavity. (Hiromichi. Y, et.al , 2019 ) The biofilm is usually initiate by Streptococcus sanguinis and Streptococcus mitins that aid the assembly of a more complex microbiota. (J. Abranches et.al , 2018) Biofilm in oral cavity form through a few process that result a structural and diversify microbial communities. (Marsh, 2006). Stage of plaque formation is initial adherence, rapid growth and detachment. (Gurenlian, 2007) In normal condition, the antagonism mechanisms between commensal and S. mutans are activated. The mechanisms are production alkaline for neutralization, generation of hydrogen peroxide as well as secretion of antimicrobial compound from the commensal such as Lactobacillus plantarum to inhibit the growth of acidogenic S. mutans in the biofilm. (J. Abranches et.al , 2018) Serotype c of S. mutans was commonly found in the oral cavity compared to serotype e, f and k. (Kazuhiko, N. & Takashi, O., 2009) However, when the cariogenic environment which is high sugar and low pH available, S. mutans rapidly ferment sugar to lactic acid that leads to a more acidic environment in the biofilm. Low pH environment decreases the survival of low acid tolerant commensal microorganisms and allow the cariogenic of S. mutans to achieve a numerical dominance. F-ATPase in S. mutans enhance the acid tolerance property by pumping the proton out of the cytoplasm and maintaining internal pH homeostasis. (Tsakalidou, 2011) Hence, cell metabolism and regulation in S. mutans is maintained. Lactic acid produced by S. mutans is capable of demineralizing enamel and cause dental caries. (Marsh, 2006) S. mutans can be transmitted horizontally and vertically. Child-nurturing habits which cause saliva transfer from adult to the children, for instance, sharing of food and utensil poses horizontal transmission of S. mutans. Vertical transmission of S. mutans occurred when breast-feeding, kissing and sleep beside the mother are carried out. Caesaran-delivery infants have higher colonization of S. mutans compared to vaginally delivered infants as Caesaran-delivery infants did not expose to the varieties of bacterium from vagina and anus. (S.G. Damle, 2016)

Everyone carried out daily oral care procedure such as tooth brushing and flossing. Some undergone tooth extraction to remove the tartar form on the surface of tooth. These activities seem to promote a good dental health, yet they may cause bacteremia to occur. Bacteremia is a condition when presence of viable bacteria in the bloodstream. S. mutans can be disseminated into the blood stream via daily oral care procedure and leads to transient bacteremia which eliminate by the host immune mechanism. (E. Christaki and E.J. Giamarellos-Bourboulis, 2014) If host immune mechanism fail or over react to the bacteria, the bacteremia becomes a bloodstream infection. Untreated bacteremia leads to more complication sucu as systematic inflammatory response syndrome (SIRS), septic shock and mutli organ dysfunction syndrome (MODS). (Smith DA, Nehring SM, 2019) Serotype of S. mutans that insolate from the patients with bacteremia have resistance to phagocytosis by polymorphonuclear leukocytes. Clinical isolation of serotype k shown similar properties, indicating serotype k strains may be causative agent for longer duration of bacteremia. (Kazuhiko, N. & Takashi, O., 2009).