Acetobacter tropicalis

Classification

domain = Bacteria
phylum = Proteobacteria
classis = Alphaproteobacteria
ordo = Rhodospirillales
familia = Acetobacteraceae
genus = Acetobacter

Introduction

Acetobacter tropicalis is a Gram-negative, rod-shaped bacterium that can be found in singles, chains, or pairs, with a size that ranges from 0.5-0.7 by 1.8-2.0 micrometers [1,2]. From past studies conducted, it has been noted that these bacteria form colonies that are circular, convex, glistening, and nonpigmented [2]. This bacteria gets its name because it is mainly found in tropical regions. The species was first found in Indonesia on coconut fruits [1,3]. This bacteria is also found to be symbiotically living with fruit flies and other insects model organisms [4]. Its colonies are positive for catalase activity, which serves for the decomposition of hydrogen peroxide into water and oxygen in host organisms [1, 2]. Overall, this bacterial species is recognized as a benign microorganism that is present in multiple areas of the environment [3]. Similar to other species from the Acetobacter genus, A.tropicalis is an obligate aerobe that can oxidize ethanol to acetic acid; oxidize acetate and lactate into carbon dioxide and water; and produce acid from glucose. This bacteria survives best in places where sugar fermentation occurs (alcoholic ecological niches), mainly in temperatures ranging from 20-37 degrees Celsius, and in pH levels of 3.5 to 8.0 [1,2]. At large, Acetobacter bacteria are fascinating due to their acid-producing and nitrogen-fixing characteristics. Most strains develop symbiotic relationships with their hosts (mainly plants and insects) [2]. However, out of all of the Acetobacter strains identified up to date, A.tropicalis stands out due to mutations that allow for this particular acetic acid bacteria (AAB) to grow above 40 degrees Celsius. In regards to recent global warming events, the ability for this strain to survive more intense temperatures is a strongly favorable characteristic [5].

Genome

In 2002, Cleenwerck et al., performed genomic studies on thirty-four different strains, representing ten species within the genus Acetobacter. Most Acetobacter strains were retrieved from different food sources — such as beer, sugar cane, sider, coffee, and multiple fruits. A. tropicalis was mainly sourced from coconut juice and lime. Most methods involved DNA-DNA hybridization experiments, DNA G+C content determination, sequencing 16S rDNA, phylogenetic analysis, and phenotypic characterization. Data analysis permitted for the identification and characterization of all ten species [6]. In particular, A. tropicalis was characterized for scoring negative in the formation of 5-Keto-D-gluconic acid and positive in the formation of 2-Keto-D-gluconic acid. In terms of growth, it scored positive for growth on glycerol and maltose, while negative in its growth on methanol. Revealing insights into the species ability to produce certain types of acid and grow in certain cabron mediums and environments. Moreover, the species scored positive for catalase activity, which helps promote a symbiotic relationship with its host via hydrogen peroxide decomposition [6]. More recently, in 2017, Wan et al. were able to isolate a strain of A. tropicalis from ''''Drosophila melanogaster''''. From these studies, the complete genome comprised a single chromosomal circle of 3,988,649 b, along with a 56% content of GC and a conjugative plasmid of 151,013 bp [7]. At large, sequencing data available comprises 10 genome assemblies. The median total length of the genome is of 3.758 Mb, the median protein count is 3182, and the median GC% is of 55.55% [9]. This information about G+C content correlates with Cleenwerck et al.’s 2002 genomic studies and assures the stability of the strain and its phylogenetic position.

Biochemical properties & distinguishable features

A. tropicalis is an obligate aerobe and therefore undergoes oxidative fermentation. Via the mechanisms of oxidative fermentation, an organism is capable of oxidizing a variety of carbohydrates to produce energy. In this process, lactic acid and ethanol can be converted into acetic acid [1,3,4]. This is usually a 2 step pathway, involving the change of ethanol to acetaldehyde and furthermore acetaldehyde into acetic acid [8]. This process is what causes acetic acid bacteria to spoil wines irreversibly [1]. Subsequently, these bacteriums can be isolated from the industrial vinegar fermentation processes and are frequently used as fermentation starter cultures [2]. In 2011, Matsutani et al. identified key features in thermotolerant ''A. tropicalis'' SKU1100. Comparative genomic studies between ''A. tropicalis'' SKU1100 and ''A.pasteurianus IFO3283-01 revealed amino acid substitutions, mainly from Lys to Arg, that could be at the basis of SKU1100 thermotolerance. Comparative protein modeling also highlighted a large number of Arg-based salt bridges in SKU1100. As more in-depth studies of SKU1100 take place, greater information could be revealed regarding the molecular mechanisms at play in thermostability [5].

Interaction with other organisms

It is important to note that contradictory results have been obtained regarding the role of microbiota in different organisms. This is mainly due to the fact that interactions and symbiotic associations between insects and their bacteria, protozoa, and fungi are complex. These relationships can shift from parasitism to mutualism and are formulated upon extracellular or intracellular interactions. At large, these interactions can have a strong influence on nutrition, physiology, or reproduction of the host [4]. A. tropicalis has mainly been found to live in insects on sugar-based diets, fruits, chocolate, and wine [1,2,3,9]. Researchers suggest that the presence of acetic acid bacteria has some sort of link to the insect sugar metabolism [4]. However, more research is still pending within this field.

References

[1] Bartowski, Eveline and Henschke, Paul. “Acetic Acid Bacteria Spoilage of Bottled Red Wine – A Review.” Journal of Food Microbiology, June 2008, pages 60-70.
[2] Sievers, Martin, and Jean Swings. “Acetobacter.” Wiley Online Library, American Cancer Society, 14 Sept. 2015, onlinelibrary.wiley.com/doi/10.1002/9781118960608.gbm00876.
[3] Hakim, Samim. “Acetobacter Tropicalis.” Viticulture and Enology, 20 Mar. 2018, wineserver.ucdavis.edu/industry-info/enology/wine-microbiology/bacteria/acetobacter-tropicalis.
[4] Kounatidis, Ilias, et al. “Acetobacter Tropicalis Is a Major Symbiont of the Olive Fruit Fly (Bactrocera Oleae).” Applied and Environmental Microbiology, American Society for Microbiology, 15 May 2009, aem.asm.org/content/75/10/3281#sec-12.
[5]Matsutani, Minenosuke, et al. “Increased Number of Arginine-Based Salt Bridges Contributes to the Thermotolerance of Thermotolerant Acetic Acid Bacteria, Acetobacter Tropicalis SKU1100.” Biochemical and Biophysical Research Communications, Academic Press, 30 Apr. 2011, www.sciencedirect.com/science/article/pii/S0006291X11007339.
[6] Cleenwerck, I, et al. “Re-Examination of the Genus Acetobacter, with Descriptions of Acetobacter Cerevisiae Sp. Nov. and Acetobacter Malorum Sp. Nov.” International Journal of Systematic and Evolutionary Microbiology, U.S. National Library of Medicine, Sept. 2002, www.ncbi.nlm.nih.gov/pubmed/12361257.
[7] Wan, Kenneth H., et al. “Complete Genome Sequence of Acetobacter Tropicalis Oregon-R-ModENCODE Strain BDGP1, an Acetic Acid Bacterium Found in the Drosophila Melanogaster Gut.” Microbiology Resource Announcements, American Society for Microbiology, 16 Nov. 2017, mra.asm.org/content/5/46/e01020-17#ref-list-1.
[8] “Acetobacter Tropicalis (ID 10668).” National Center for Biotechnology Information, U.S. National Library of Medicine, www.ncbi.nlm.nih.gov/genome/?term=Acetobacter%2Btropicalis%5BOrganism%5D&cmd=DetailsSearch.
[9] “Acetobacter Tropicalis – Stiven Mita.” How Microbes Create Our Favorite Delicacies, 28 Nov. 2016, fermentationstations.wordpress.com/2016/11/01/acetobacter-tropicalis-stiven-mita/.