In the years 2006-2011 a total of 617 Proteus spp. strains were collected in Łódź, Poland (Fig. 1) including the 24 clinical strains reported earlier [13-20]. What is worth noticing, no strains were collected from blood, although Proteus spp. bacteria may also cause severe bacteremia and sepsis [2, 27]. Urine and wound isolates were prevailing especially among the ubiquitous P. mirabilis. Frequent isolation of P. mirabilis (Table 1) is not surprising, as the species is regarded as the most virulent and widespread species in the genus Proteus, especially in patients suffering from nosocomial urinary tract and wound infections and it is still the most frequently isolated species among Proteus spp. [2]. P. genomospecies 4, 5, 6, or P. hauseri are not usually distinguished from P. vulgaris in the routine laboratory work and so there are very few reports available on their occurrence in patients. In comparison, O’Hara et al. [4] and Janda et al. [5] studying several P. hauseri, P. genomospecies 4, 5, and 6 strains, mostly from human sources, found P. genomospecies 5 and 6 to be the most numerous. Our result confirm the low frequency of P. penneri isolation from clinical sources [28, 29]. R forms which are regarded as less virulent, were found among the isolates belonging to all the identified Proteus species. However, in the most pathogenic P. mirabilis species, the smallest number of R forms was detected and the best swarming ability was noticed. The feature of swarming is considered to be an important virulence factor of P. mirabilis bacteria and may be a form of their adaptation to the host environment (especially urinary tract). Moreover, P. mirabilis swarming cells facilitate an entry into the urinary tract of other non-motile species, which often leads to following polymicrobial UTIs [30]. P. vulgaris isolates demonstrated weaker swarming growth in comparison to P. penneri strains, similarly to the results achieved by Kwil et al. [30].
Serological studies confirmed that the studied strains possess various core types which is typical in the genus Proteus. Frequently, strains forming a common R serogroup do not belong to a common O serogroup and the strains classified into one O serogroup may differ in their core-region serospecificity [6, 11]. Thus, the use of antisera which are rich in core-specific antibodies is informative, but in the studies aimed at classifying Proteus spp. strains into proper O serogroups may be misleading. [19]. A good solution might be the use in the preliminary classification studies of the antisera deprived of core-specific antibodies removed by adsorption, which is hard to obtain, or the application of further detailed tests allowing a precise serological classification of strains on the basis of their O antigens serospecificity, as presented in this work. Among the studied strains, 13 isolates preliminarily classified into the respective predominating O serogroups were further excluded on the basis of subsequent detailed studies.
Our studies showed a big serological diversity among Proteus spp. strains isolated recently from patients in central Poland. However, half of the isolates (299) including 18 strains reported before [13-15, 19] belong to 15 most numerous O serogroups, comprising ten or more studied strains (Fig. 5). O78 serogroup is the biggest one, formed by 61 studied strains (10.5% of 580 collected S strains). This serogroup together with O77 and O79 serogroups were described as new ones not long ago by Drzewiecka et al. [13, 14] and Arbatsky et al. [15] and they currently include only the isolates from Poland. O50 serogroup, in turn, was reported in 2003 [32] and has so far been represented by P. mirabilis strain TG 332 described earlier as being serologically unique [33]. O60 serogroup was also formed in 2003 [34] for a non-clinical P. myxofaciens isolate from a gypsy larva. Lately, the species has been proposed to be excluded from the genus Proteus [35]. However, our results clearly indicate that O60 serotype exists among P. mirabilis and P. genomospecies clinical strains. The other 10 most prevalent serogroups have been included in the first classic serological scheme proposed for P. mirabilis and P. vulgaris bacteria [9]. What is interesting, the majority of these predominant serogroups had been previously reported as frequently found among hundreds of clinical P. mirabilis and P. vulgaris isolates from urine, faeces, blood, or unknown sources. Summarizing these data, Larsson [10] indicated O3 serogroup as dominating in all reports and O10, O13, O26, O28, and O30 serogroups as the most prevalent ones. O6, O11, O23, O24, O27, and O29 serogroups were also widely distributed. Analyzing the serological properties of 99 Swedish and 24 Polish P. mirabilis strains from urinary tract infections, Kaca et al. [12] also found O10 and O30 to be the most numerous serogroups. However, the authors applied only 20 Proteus O-specific sera in their studies so the attachment of the strains to the other prevalent serogroups (e.g. O6, O26, O28, or O29) was not analysed. Research on P. penneri strains revealed that clinical strains and isolates from faeces of healthy people in the USA, Great Britain, Canada, France, Germany, and single strains from other countries belonged mostly to O17, O61, O64, and O65 serogroups [11, 36] including also P. mirabilis and/or P. vulgaris strains [7]. In the presented studies three P. penneri isolates were classified into O10 serogroup. Several P. genomospecies strains were classified into O11, O60, O78, and O79 serogroups and two P. vulgaris isolates into O27 and O78 serogroups. Thus, some of the prevalent serogroups turned out to be multi-species (Table 3), including not only the ubiquitous P. mirabilis but also the other species.
O78, O16, and O11 serogroups seem to be heterogenous. Further chemical analysis may show the differences in the structures of O antigens, responsible for their slight serological variety observed within these serogroups and will allow the formation of new subserogroups or new cross-reacting O serogroups.
It can be seen that for many years, Proteus spp. strains belonging to O3, O6, O10, O11, O27, O28, and O30 serogroups have been most frequently isolated in many different countries. What is worth noticing, the Proteus O60 antigen structure resembles that of the Proteus O30 antigen [7] so before the O60 serogroup was created in 2003 [34], O60 isolates could be classified to O30 serogroup due to their mutual similarity and strong cross reactivity revealed in our work (data not shown). It should be noted that at present, O78 serogroup obviously seems to be the most prevalent among Polish patients from the Łódź region accounting for 10.5% of all S isolates, although its predominance is not big. The prevalence of the serotypes dominating over others may be connected with some features of their O antigens. However, it is difficult to indicate common structural O-PS features (Table 3) distinguishing the biggest serogroups from the smaller ones, e.g. the compounds present exclusively in the frequent O antigens and, simultaneously, absent in the rarely found O serotypes [7]. Among the 15 most numerous serotypes, both branched and linear O-PS repeating units can be found, which are built of three-to-five sugars, including glucose, galactose, and their derivatives frequently occurring in Proteus O-PSs. The rest of 2-amino-2-deoxy-D-glucose (D-GlcN) is the most frequent constituent building all the dominating serotypes except O3, however, it is also the most widely distributed component of Proteus O serotypes [7]. Some of these frequent O antigens contain also rare sugars like altruronic acid or deoxytalose and other compounds like amino acids or ribitol phosphate and all of them are acetylated. None of these features is exceptional or atypical for Proteus O antigens. For example, the repeating unit of the most ubiquitous O78 antigen is composed of five different sugars and sugar derivatives. Three of them constitute the main chain and two form side branches. This antigen is not characterized by any unusual or unique structural feature. It shares disaccharide fragments with several Proteus O antigens, but no cross reactions have been noticed [14].