ACETIC ACID BACTERIA, BIOTECHNOLOGICAL APPLICATIONS LUCIANA DE VERO,MARIA GULLO, and PAOLO GIUDICI Department of Agricultural and Food Sciences, University of Modena and Reggio Emilia, Reggio Emilia, Italy DESCRIPTION AND SIGNIFICANCE The occurrence of acetic acid bacteria (AAB) in nature (flowers, plants, and fruits) and in food ecosystems (vine- gar, wine, beer, and others) as well as their exploita- tion in biotechnological applications (synthesis of com- pounds with high economic relevance) reflect the general metabolism of these bacteria, based on the incomplete oxi- dation of several carbohydrates and alcohols. Recently, the discovery of a pathogenic member opened a new scenario in AAB group definition and significance. In Table 1, cur- rently recognized genera and species are reported (1). One of the most important aspects related to the AAB study is the difficulty in the strains’ isolation and preservation. The nature of the uncultivable status of AAB cells out- side the natural environment is not well known and until now research mainly has focused on the setup of media and optimal growth conditions of culturable strains (2–4) without knowing the metabolic difference between strains under controlled conditions and those in the environ- ment. A promising perspective could be the understanding of AAB intercellular communication system which regu- lates the transcription of specific target genes in a cell density-dependent manner. These cell–cell communica- tions, termed quorum sensing, allow bacteria to moni- tor their cell density by measuring the concentration of self-produced diffusible signal molecules (autoinducers). Several physiological regulation and functions, including secondary metabolite production, swimming and swarm- ing motility, conjugal plasmid transfer, biofilm formation and virulence are mediated by quorum sensing. In AAB, recently it was stated that Gluconacetobacter intermedius strains contain an N-acylhomoserine lactone-based quo- rum sensing system, responsible for the control of the expression of the gene (ginA), which in turn represses oxidative fermentation, including acetic acid and gluconic acid fermentation (5). Taxonomy In the last decade the AAB taxonomy has known important advances as a result of the contribution of the polyphasic approach, allowing for a more comprehensive knowledge of the information derived from metabolism, ecology, genome characterization and phylogeny (6,7). Nowadays, under the generic name ‘‘acetic acid bacteria’’ 10 genera of the Acetobacteraceae family are grouped (Class: Encyclopedia of Industrial Biotechnology: Bioprocess, Bioseparation, and Cell Technology, edited by Michael C. Flickinger Alphaproteobacteria): Acetobacter (A.), Acidomonas (Ac.), Asaia (As.), Gluconacetobacter (Ga.), Gluconobacter (G.), Granulibacter (Gr.), Kozakia (K.), Neoasaia (N.), Saccharibacter (Sa.), and Swaminathania (S.) and one genus, Frateuria (F.), of the family Xanthomonadaceae (Class: Gammaproteobacteria). The taxonomic dynamicity of AAB over the years is well documented in literature mainly resulting in changes in the classification, nomenclature and recognition of new species (8). In spite of the major efforts of taxonomists to provide accurate and up-to-date information, misunderstanding due to uncor- rected nomenclature uses and misidentification of strains often occurs. In this chapter the AAB species’ names are reported according to the current classification and the taxonomic changes recognized by the International Code of Nomenclature of Bacteria involving Acetobacter, Gluconacetobacter, Acidomonas, Gluconobacter and Frateuria genera are listed in Table 2 (1). Currently, the main drawbacks in taxonomy and genome structure derive from the lack of full genome sequences of type strains. Another bottleneck is related to the phenomenon of strains evolution over the time that affects bacterial species designation. However, currently, there is no answer to deal with the microbial change during the time in laboratory conditions and this has crucial consequences in clustering bacteria according to phenotypic traits. Regarding AAB, some authors observed the loss of physiological activities as consequence of spontaneous mutations. For instance, Kondo and Hori- nouchi (9), studied spontaneous high-frequency mutations resulting from insertion sequences (IS) elements, that are responsible for genetic instability leading to deficiencies in various physiological properties of AAB, such as ethanol oxidation and cellulose production. Genome Features Hitherto, only the whole genome of Gluconobacter oxy- dans (strain 621H, DSMZ 2343) (10) and Granulibacter bethesdensis (strain CGDNIH1, ATCC BAA-1260T=DSM 17861T) (11) have been sequenced (Table 3). G. oxydans is recognized as the most important for biotechnological purposes and Gr. bethesdensis is the first agent of the invasive chronic granulomatous disease and also the first pathogenic AAB member (11). The genome sequence of G. oxydans has provided detailed insights into the oxidative potential and elucidated the mechanism of the incomplete oxidation. The core system of the respiratory chain of G. oxydans consists of a non-proton-translocating NADH: ubiquinone oxidoreductase and two quinol oxidases of the bo 3 and bd type, respectively (10). In the G. oxydans genome a large number of repeated DNA elements, which are known to be involved in genomics rearrangements were detected. In particular, 82 IS and 103 transposase genes were identified. Some of these copies are partially deleted and therefore presumed defective. Comparative analysis of G. oxydans and Gr. bethesdensis genomes reveals remarkable features of 1 Copyright  2010 John Wiley & Sons, Inc.