Bacteria from the Chlamydiales order have been long known, especially as pathogenic bacteria to humans and many animal species, principally including birds and mammals. But for slightly over 20 years, they have been identified in the aquatic environment as endosymbionts of amoebas and sea worms. For several years, they have also been recorded as a cause of diseases among fish, causing respiratory system infections in the form of epitheliocystis of the gill. At present, 11 chlamydia-like organisms pathogenic to fish have been described, including nine new ones, classified into six families, four of which are already known (Parachlamydiaceae, Rhabdochlamydiaceae, Candidatus Parilichlamydiaceae, Candidatus Clavichlamydiaceae) and two newly created families, namely Candidatus Actinochlamydiaceae and Candidatus Parilichlamydiaceae. This paper characterises 11 chlamydia-like organisms, as well as seven isolates not classified into families, which are pathogenic to fish, presenting their genetical properties allowing for their classification, as well as morphological properties and diseases caused.

Chlamydia are absolute pathogens of humans and animals; despite being rather well recognised, they are still open for discovery. One such discovery is the occurrence of extrachromosomal carriers of genetic information. In prokaryotes, such carriers include plasmids and bacteriophages, which are present only among some Chlamydia species. Plasmids were found exclusively in Chlamydia (C.) trachomatis, C. psittaci, C. pneumoniae, C. suis, C. felis, C. muridarum and C. caviae. In prokaryotic organisms, plasmids usually code for genes that facilitate survival of the bacteria in the environment (although they are not essential). In chlamydia, their role has not been definitely recognised, apart from the fact that they participate in the synthesis of glycogen and encode proteins responsible for their virulence. Furthermore, in C. suis it was evidenced that the plasmid is integrated in a genomic island and contains the tetracycline-resistance gene. Bacteriophages specific for chlamydia (chlamydiaphages) were detected only in six species: C. psittaci, C. abortus, C. felis, C. caviae C. pecorum and C. pneumoniae. These chlamydiaphages cause inhibition of the developmental cycle, and delay transformation of reticulate bodies (RBs) into elementary bodies (EBs), thus reducing the possibility of infecting other cells in time. Plasmids and bacteriophages can be used in the diagnostics of chlamydioses; although especially in the case of plasmids, they are already used for detection of chlamydial infections. In addition, bacteriophages could be used as therapeutic agents to replace antibiotics, potentially addressing the problem of increasing antibiotic-resistance among chlamydia.

Pathogenic bacteria cause many diseases, some of which are fatal. For researchers, it is a challenge to understand bacterial mechanisms of pathogenicity, including their virulence pathways regulated by RNA. This work presents data on the mechanisms of regulation and expression of several virulence factors coded by RNA, namely 5' UTR fragments, riboswitches and small non-coding RNA (sRNA).

More and more attention has been paid to environmental chlamydiae in recent years. They were classified as pathogenic bacteria for both humans and animals. Thanks to molecular biology techniques, the following nine families of environmental chlamydiae were assigned to the order of Chlamydiales: Candidatus Clavichlamydiaceae, Criblamydiaceae, Parachlamydiaceae, Candidatus Piscichlamydiaceae, Rhabdochlamydiaceae, Simkaniaceae, Waddliaceae, Candidatus Actinochlamydiacae and Candidatus Parilichlamydiaceae. These bacteria are considered the infectious factors of zoonoses due to the fact that they can be found among pets and livestock such as cats, guinea pigs, sheep, cattle, and even fish. Many of these animals also suffer from diseases caused by these bacteria. In this study, while characterizing environmental chlamydiae, special attention has been paid to illnesses of the respiratory tract caused by Simkania negevensis, and to the abortions among people and ruminants caused by Waddlia chondrophila. Furthermore, the species of the Rhabdochlamydiaceae family , as well as the bacteria from the Parachlamydiaceae family responsible for eye illnesses in humans and animals, are also characterized in this work. Lastly, newly discovered fish chlamydiae, which are potential factors of illnesses in humans and terrestial animals, are also presented in this paper.

Chlamydiae are bacteria commonly found in the environment of human life and they cause many diseases. Chlamydial infections are common among men and women, although they are more often diagnosed in women and they are frequently asymptomatic, due to the biology of chlamydiae. Incorrectly diagnosed and untreated, they cause many complications which can lead to infertility in both, men and women. This work presents the diseases caused by chlamydiae of the urogenitary tract, including environmental chlamydiae, so far in Polish literature rarely presented, which extends the panel responsible for chlamydial genital tract disease, particularly in women. A major problem is that the knowledge of the immune mechanisms that occur in the case of infection with these bacteria, especially in view of prevention of these infection, mainly in the context of creation of the vaccine. Also important fact from the knowledge of immunity in chlamydiosis is base for diagnosis of these infection.