Once mycorrhizal colonisation has occurred, subsequent exudation release by the root may be modified both through the mycorrhizal fungus acting as a considerable carbon sink for photoassimilate and through hyphal exudation. This may be expected to lead to changes in both the qualitative and quantitative release of exudates into the mycorrhizosphere. AM colonisation generally decreases root exudation  although not always  and may be influenced by the species of fungus present . In addition, a reduction in sugar and amino acid release has been reported in some studies [19,22] but there is no clear pattern as to the consistency of this phenomenon. Similarly the reported impact on the mycorrhizosphere community is equally inconsistent with, for example, fluorescent pseudomonads showing a decrease, increase or no effect following AM colonisation [10,21,23,24]. Meyer and Linderman  observed no alteration in the total number of bacteria or actinomycetes isolated from the rhizosphere of Zea mays and Trifolium subterranean L. colonised by the AM fungus Glomus fasciculatum. However, there was a change in the functional groups of these organisms including more facultative anaerobic bacteria in the rhizosphere of AM colonised T. subterraneum but fewer fluorescent pseudomonads and chitinase-producing actinomycetes in the rhizosphere of AM-colonised Z. mays. The total number of bacteria isolated from the rhizoplane of both T. subterraneum and Z. mays increased as a result of AM colonisation although total numbers of actinomycetes were unaffected. In addition, leachates from Z. mays rhizosphere soil reduced production of zoospores and sporangia by Phytophthora cinnamomi when colonised by G. fasciculatum than non-mycorrhizal Z. mays rhizosphere leachates indicating a potential mechanism by which AM colonisation may aid pathogen resistance . However, the chitinolytic producing actinomycete population may act as general biocontrol agents, thus, the reduction in this population may mean chitin containing pathogens become more important. Using three different AM fungi (Glomus etunicatum, Glomus mosseae or Gigaspora rosea) Schreiner et al.  observed differences in bacterial groups (i.e. Gram-negative or Gram-positive) depending on which fungus had colonised the roots of Glycine max L. (soybean). The AM fungus G. mosseae produced the greatest amount of external hyphae (i.e. 8.1 m g−1 soil). The other two AM fungi did not differ in the amount of external hyphae they produced but soil sampled from pots containing G. etunicatum had higher amounts of Gram-positive bacteria, measured as colony-forming units per g of dry soil, than corresponding samples from G. rosea. Soil from G. etunicatum pots also contained higher counts of Gram-negative bacteria than those counted from G. mosseae. These results would seem to imply that the hyphosphere (the volume of soil influenced by the external mycelium of the AM fungus) of different AM fungi may influence certain bacterial groups however, it should also be noted that where external mycelium production was greatest (i.e. in the case of G. mosseae) the influence on overall counts was less than from G. etunicatum which produced a less extensive mycelium. Indeed, low P status of the soil had a greater effect on total bacterial, and in particular Gram-positive, counts, than did mycorrhizal treatments. Alternatively, the more extensive mycelium could have inhibited bacterial populations as a means of reducing competition for nutrients in the mycorrhizosphere [15,24]. Other studies specifically testing the hyphosphere soil have found no quantitative change in bacterial numbers [27,28]. However, whereas Andrade et al.  found variations in bacterial composition which depended on the AM fungus present, Olsson et al.  found no such changes in composition or activities of the bacterial community.
Filion et al.  examined the release of soluble unidentified substances by the external mycelium of Glomus intraradices on the conidial germination of two fungi and the growth of two bacteria. Conidial germination of Trichoderma harzianum and growth of Pseudomonas chlororaphis were stimulated whereas growth of Clavibacter michiganensis subsp. michiganensis was unaffected and conidial germination of Fusarium oxysporum f.sp. chrysanthemi was reduced. These observed effects were generally correlated with the extract concentration. The authors  suggested that this was a possible means by which the AM mycelium may alter the microbial environment so that it was detrimental to pathogens. In contrast, Green et al.  also examining the interaction between G. intraradices and T. harzianum, observed no effect of the AM external mycelium on the population density of T. harzianum, except in the presence of an organic substrate when population densities and metabolic activity of T. harzianum were actually reduced. The differing results reported on the influence of AM fungi upon soil micro-organisms therefore are probably not only due to the type of AM fungus present but also the conditions, such as soil nutrient availability, in which the interaction is studied.