This question can not be answered purely theoretically. Especially, with an emphasis on your sub-question "Organic farming friend or enemy?" It is evident that every process has both good and bad sides. If we just remember recent contaminations with pathogens from organically-grown vegetables. Though, PGPR is a specific thing. I suggest to start with a list of putative PGPRs, then try to find any possible health-related issues in respect to particular taxa.

I don't think many PGPRs are harmful to human health when grown on the roots of colonized plants. Some may produce whatever compound (toxin?) that may get distributed systemically but it is very likely that they activate plant enodenous mechanisms in the root that trigger the beneficial effect(s). No idea however if in direct contact or incorporation of these guys.

Dear Gederts,Dear Uwe,Thank you for your coments.There is no doubt that PGPRs stimulate growth of the plants and grow on the roots of colonized plants.But I think we are not allowed to use everything(with using high volume of PGPRs around roots zoon) that promot plant growth but with bad effect on human health.If we have a short search in the web we can find many of the PGPRs among the list of bioweapon welfares(Agroterrorism).For example Bacillus,Peusodomonas and Herbaspirilum species.The problem is here that agronomist work in their fields and medical experts work in their labs and they have not joint conferences to share their experience.In organic farming systems farmers spray or use these small killers(PGPRs)through irrigation systems or inoculate them with seeds and seedlings and consumers eat vegtables and other cereals etc.So people will be contaminated with these bacteria.Most of these PGPRs can be lived in the human body temperature(37 0C).Most of the animals and birds in the environment can be contaminated aswell.The dangerous part is here that the genome of the bacteria may mix with the genome of humans.So I fear no antibiotic affect on these bacteria in the body of humans in the future.

I think if we are doing in the correct way on screening of the PGPR, then the negative effect for human health could be minimized. We have to sure that the selected PGPRs do not belong to pathogen one, it should be proofed by molecular taxonomy analysis. Based on the molecular taxonomy analysis, then check in the data base "Risk Group Classification". I would suggest to use PGPRs that belong WHO Risk Group 1 (no or low individual and community risk), other ways if the PGPRs belong to higher RG must be checked for biosafety test.

Dear all,

here are some specific cases which may help to find an answer to the question: Pantoea agglomerans is a plant growht promoting microbe (PGPM) that improves the growth of maize [1] but causes infections in soft tissue or bone/joint in humans [2]. Also, Burkholderia species have several benefits in the filed (e.g., biological control of plant pathogens, bioremediation, plant growth promotion), but some Burkholderia species have been involved in human infections [3]. On the other hand, some recent studies show that there can be ingress of human pathogens into crop plants [4], but that this process is mediated bz soil properties [5].

In conclusion, I think finding agriculturally appropriate PGPM that are safe for humans will depend on (i) a thourough screening programme and (ii) a better understanding of the ecological mechanisms involved.

[1] Riggs PJ, Chelius MK, Iniguez AL, Kaeppler SM and Triplett EW (2001): Enhanced maize productivity by inoculation with diazotrophic bacteria. Australian Journal of Plant Physiology 28:829-836.

[2] Cruz AT, Cazacu AC and Allen CH (2007): Pantoea agglomerans, a plant pathogen causing human disease. Journal of Clinical Microbiology 45:1989-1992.

[3] Coenye T and Vandamme P, (2003): Diversity and significance of Burkholderia species occupying diverse ecological niches. Environmental Microbiology 5:719-729.

[4] Gu G, Cevallos-Cevallos JM and van Bruggen AHC (2013): Ingress of Salmonella enterica Typhimurium into tomato leaves through hydathodes. PLoS ONE 8:e53470.

[5] Gu G, Cevallos-Cevallos JM, Vallad GE and van Bruggen AHC (2013): Organically managed soils reduce internal colonization of tomato plants by Salmonella enterica serovar Typhimurium. Phytopathology 103:381-388.

Dear Sarjiya, Thank you very much for your helpfull comments.Dear Thomas, Thank you for your comments and very nice references.I appreciate all respected researchers who give their opinion to my question.

Just don't forget that these bacteria were allways in the soil in the first place!

steping upon millions of klebsiella strains is something you do everyday (as long as you walk outside concrete and asphalt for a minute). Most PGPR used for innoculation came form usual environments (farms, crops and fields) where farmers were not sistematicly ill due to soil bacteria inffections (or else farming would be a deadly activity). Of course care should be taken before industrial release of any biological product, but it is not like PGPR innoculation brings fearsome biological agents that the plant-soil-farmer environment has never seen before.

I would support opinion of Dr. Doring. Many potential PGPR are related to opportunistic pathogens of animals and humans, and strains for practical application must be tested carefully before wide-scale application. But risk of infection by PGPR for healthy person is very low - about the same as after outdoor camping.