The Ixodes scapularis Symbiont Rickettsia buchneri Inhibits Growth of Pathogenic Rickettsiaceae in Tick Cells: Implications for Vector Competence

Abstract

Ixodes scapularis is the primary vector of tick-borne pathogens in North America but notably does not transmit pathogenic Rickettsia species. This tick harbors the transovarially transmitted endosymbiont Rickettsia buchneri, which is widespread in I. scapularis populations, suggesting that it confers a selective advantage for tick survival such as providing essential nutrients. The R. buchneri genome includes genes with similarity to those involved in antibiotic synthesis. There are two gene clusters not found in other Rickettsiaceae, raising the possibility that these may be involved in excluding pathogenic bacteria from the tick. This study explored whether the R. buchneri antibiotic genes might exert antibiotic effects on pathogens associated with I. scapularis. Markedly reduced infectivity and replication of the tick-borne pathogens Anaplasma phagocytophilum, R. monacensis, and R. parkeri were observed in IRE11 tick cells hosting R. buchneri. Using a fluorescent plate reader assay to follow infection dynamics revealed that the presence of R. buchneri in tick cells, even at low infection rates, inhibited the growth of R. parkeri by 86–100% relative to R. buchneri-free cells. In contrast, presence of the low-pathogenic species R. amblyommatis or the endosymbiont R. peacockii only partially reduced the infection and replication of R. parkeri. Addition of host-cell free R. buchneri, cell lysate of R. buchneri-infected IRE11, or supernatant from R. buchneri-infected IRE11 cultures had no effect on R. parkeri infection and replication in IRE11, nor did these treatments show any antibiotic effect against non-obligate intracellular bacteria E. coli and S. aureus. However, lysate from R. buchneri-infected IRE11 challenged with R. parkeri showed some inhibitory effect on R. parkeri infection of treated IRE11, suggesting that challenge by pathogenic rickettsiae may induce the antibiotic effect of R. buchneri. This research suggests a potential role of the endosymbiont in preventing other rickettsiae from colonizing I. scapularis and/or being transmitted transovarially. The confirmation that the observed inhibition is linked to R. buchneri's antibiotic clusters requires further investigation but could have important implications for our understanding of rickettsial competition and vector competence of I. scapularis for rickettsiae.

Cryptic Genes for Interbacterial Antagonism Distinguish Rickettsia Species Infecting Blacklegged Ticks From Other Rickettsia Pathogens

“…While previously observed in ticks ( Burgdorfer et al., 1981 ; Macaluso et al., 2002 ; Wright et al., 2015 ; Levin et al., 2018 ) and cell culture ( Cull et al., 2022 ), the ability of Rickettsia infection to block Rickettsia superinfection remains a sorely understudied aspect of vector biology. Our identification and characterization of rCRCT/CRCA modules adds to a short list of factors, namely BOOM, AGAB and PKS-NRPS, that have been hypothesized to underpin a mutualism between R .…”

“…Future characterization of these factors will determine their contribution to blocking superinfection of I . scapularis by Rickettsia pathogens ( Cull et al., 2022 ). When mutagenesis is someday an efficacious tool for bioengineering Rickettsiae, this line of research will offer a gene drive tool ( R .…”

Cryptic Genes for Interbacterial Antagonism distinguish Rickettsia species Infecting Blacklegged Tick from other Rickettsia pathogens

“…While previously observed in ticks (Burgdorfer et al, 1981;Macaluso et al, 2002;Wright et al, 2015;Levin et al, 2018) and cell culture (Cull et al, 2022), the ability of Rickettsia infection to block Rickettsia superinfection remains a sorely understudied aspect of vector biology. Our identification and characterization of rCRCT/CRCA modules adds to a short list of factors, namely BOOM, AGAB and PKS-NRPS, that have been hypothesized to underpin a mutualism between R. buchneri and blacklegged tick.…”

“…Our identification and characterization of rCRCT/CRCA modules adds to a short list of factors, namely BOOM, AGAB and PKS-NRPS, that have been hypothesized to underpin a mutualism between R. buchneri and blacklegged tick. Future characterization of these factors will determine their contribution to blocking superinfection of I. scapularis by Rickettsia pathogens (Cull et al, 2022). When mutagenesis is someday an efficacious tool for bioengineering Rickettsiae, this line of research will offer a gene drive tool (R. buchneri) ready to disseminate into blacklegged tick populations to combat the spread of human disease agents, Yang, Q., Kučerová, Z., Perlman, S. J., Opit, G. P., Mockford, E. L., Behar, A., et al subjects retrieved from a Blastp search against the NCBI nr database using five concatenated R. buchneri proteins (REIS_1427-REIS_1423) as the query (further information provided in Fig.…”

“…More recently, the PKS NRPS module and the AGAB synthesis/resistance gene array were investigated for their possible roles in limiting superinfection of pathogenic rickettsia in tick cells infected with R. buchneri str. ISO7 (Cull et al, 2022). Despite demonstrating that R. buchneri substantially reduced superinfection by pathogenic R. parkeri in cell culture, no anti-bacterial activity against either R. parkeri or extracellular bacteria (Escherichia coli and Staphylococcus aureus) was shown by the unknown product(s) of these loci, leaving their function in R. buchneri unclear.…”