Isolation and characterization of atypical <i>Actinobacillus pleuropneumoniae</i> serovar 15 lacking the <i>apxIICA</i> genes in Japan

Abstract

Six atypical Actinobacillus pleuropneumoniae serovar 15 strains were isolated from pneumonic lesions of naturally infected dead pigs from the same farm in Japan. Genetic analyses of apx genes revealed that the atypical isolates contained the toxin-associated genes apxIBD, apxIIICA, apxIIIBD, and apxIVA, but not apxIICA. Coinciding with the result of the atypical gene profile, analyses of toxin protein production revealed that these atypical isolates expressed only ApxIII but not ApxII. A mouse pathogenicity test showed that the atypical isolate tested seemed to be less virulent than the typical isolates. This is the first report describing the emergence of atypical A. pleuropneumoniae serovar 15, which does not produce ApxII due to the absence of apxIICA genes, in Japan.

Sero- and apx-typing of German Actinobacillus pleuropneumoniae field isolates from 2010 to 2019 reveals a predominance of serovar 2 with regular apx-profile

“…As a consequence of the repeatedly reported detection of atypical isolates in A. pleuropneumoniae [ 17 – 21 ], an additional aim of this study was the characterization of the apx -profiles in order to check the validity of the alleged serovar-specific association of these toxin genes. Among others, strains with atypical toxin gene profiles were found in Switzerland, Korea, Japan and Australia, belonging to a wide range of different serovars [ 17 – 21 ]. The toxin typing performed here revealed that only 7% of the examined isolates deviated from the expected apx -profile.…”

“…The description of the serovar-specific production of Apx toxins dates back to the 1990s. Since then, however, reports of atypical isolates of the bacterium have been published repeatedly [ 17 – 21 ] but the relevance of these findings is unclear.…”

Genetic Diversity of Actinobacillus pleuropneumoniae Serovars in Hungary

“…Serovars carrying apxIII together with apxII are serovars 2, 4, 6, 8, and 15; they are also considered virulent, i.e., traditionally, high virulence is attributed to the presence of at least two secreted exotoxins (ApxII plus ApxI or ApxIII) besides the intrinsically produced ApxIV, while biotypes/serovars carrying only one toxin gene besides ApxIV are considered to show low virulence, thus they are unlikely to cause epizootics [ 3 ]. In addition to typical toxin-producing strains, atypical ones lacking certain genes also occur [ 26 ]. This virulence-based grouping is in line with the lipopolysaccharide O-chain or omlA phylogroups as well [ 3 , 27 ]; however, other virulence factors also contribute to disease severity [ 12 ].…”

Characterization of nontypeable Actinobacillus pleuropneumoniae isolates

“…Further examination of the genes for the expression, activation, and secretion of Apx toxins by PCR 1,17 revealed that these 2 isolates of A. pleuropneumoniae , named IB19-1 and IB19-2, contained the same apx gene combination as serovars 2, 4, 6, 8, and 15 (an apxIICA, apxIIICA, apxIBD, apxIIIBD , and apxIVA gene pattern). The serovar 15 reference strain HS143 1 and 3 representative field isolates (A–C) 14 of serovar 15 isolated from different geographic regions in Japan were used in the further characterization of these isolates as described below.…”

“…Template DNA was prepared as described previously. 14 Briefly, the conditions for PCR were as follows: 94°C for 1 min (1 cycle), 98°C for 10 s, and 68°C for 3 min 30 s (30 cycles). The PCR products were purified (NucleoSpin gel, PCR clean - up kit; Macherey-Nagel), and the nucleotide sequences were determined using a primer-walking procedure as described previously.…”