1 Section of Microbiology and Center for Genetics and Development, University of California, One Shields Avenue, Davis, CA 95616, USA
2 USDA Agricultural Research Station, Salinas, CA 93905, USA
Correspondence
Mitchell Singer
mhsinger{at}ucdavis.edu
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ABSTRACT |
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INTRODUCTION |
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Predation, however, represents only part of the M. xanthus life cycle. In the absence of prey or suitable nutrients, the M. xanthus cells within a dense, starving colony cooperate to form a macroscopic fruiting body, within which 110 % of the cells differentiate into resistant myxospores (Dworkin, 1996). The execution of this developmental program is dependent on a number of regulatory components implicated in the sensing of environmental signals, including the developmental regulators AsgA (Plamann et al., 1995
), CsgA (Kim & Kaiser, 1990
; Shimkets & Rafiee, 1990
) and SdeK (Garza et al., 1998
; Kroos et al., 1990
).
In addition, since the formation of fruiting bodies is dependent on the movement of cells towards aggregation centres, mutations in many motility genes also cause deleterious effects on development (Hodgkin & Kaiser, 1979a, b
; MacNeil et al., 1994b
; Zusman, 1982
). M. xanthus possesses two separate motility mechanisms, termed adventurous (A) motility and social (S) motility. The A-motility system mediates the spatial translocation of both individual and small groups of cells, while the S-motility system governs the contact-dependent movement and coordination of swarms of cells (Hodgkin & Kaiser, 1979a
, b
; MacNeil et al., 1994a
, b
). S mutations cause greater defects in fruiting-body morphology than mutations in the A system, which suggests that proficiency for S motility is more important for development (MacNeil et al., 1994a
, b
). In addition, the Frz system, which comprises components that are homologous to the Che proteins involved in chemotaxis of Escherichia coli (Blackhart & Zusman, 1986
; McBride et al., 1989
; Trudeau et al., 1996
), regulates the developmental process by controlling the reversal frequencies of individual cells (Shi et al., 1996
).
Since local nutrient levels dictate entry into either the developmental or predatory modes of the M. xanthus life cycle, it is likely that mutations affecting entry into one mode might also impact entry into the other. Many techniques exist for determining the effect of various mutations on the organism's ability to go through the developmental cycle. As a result, a substantial body of information exists for the genetics of M. xanthus fruiting body formation, sporulation and motility (for reviews, consult Kaiser, 2003; Søgaard-Andersen et al., 2003
). Here, we contribute another piece to the proverbial puzzle by assessing the predatory capacity of mutants defective for various processes in both development and related motility systems.
In order to carry out this study, we developed a method for quantifying predation by M. xanthus. S. marcescens was selected as the primary prey species in a new assay described here, because it is (i) simple to culture, (ii) susceptible to lysis by M. xanthus (Singh & Yadava, 1976), (iii) easily imaged due to the striking contrast between its red (prodigiosin-derived) pigmentation and the yellow colour of M. xanthus (Williams & Qadri, 1980
), and (iv) accessible to other myxobacteriologists given its relatively low hazard potential to human health and agriculture (for a review, consult Hejazi & Falkiner, 1997
). In addition to this assay, in which predation by M. xanthus on live prey is obligatory for sustaining growth, we also employed a previously described assay that measures predation of prey bacteria in the presence of limited nutrients (Bull et al., 2002
). Both assays were used in this study since they measure predation under differing conditions, and, therefore, a comparison of their results might offer further insights into the genetic requirements for predation when such conditions are encountered in the soil.
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METHODS |
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Since differences in motility among the M. xanthus strains could affect predation results using the lawn assay, we devised the following method to account for potential motility differences. Swarming on nitrocellulose in the presence of non-prey nutrients yeast extract and proteolytic digests of casein in this case was determined by briefly immersing 13 cm circles of nitrocellulose membrane in CTTYE, then laying them atop CTTYE agar, and finally comparing these to assays conducted in the presence of prey bacteria (as described above). Wild-type and mutant strains were spotted onto the same nutrient filter, allowed to grow for about 7 days, and the swarm sizes imaged and quantified as above.
Streak predation assay.
The streak predation assay was conducted according to previously published methods (Bull et al., 2002). Briefly, bacterial cultures to be tested as prey (Table 2
) were suspended in 100 µl TPM buffer to a density of 0·6 OD600 units, using a spectrophotometer (UV-1601, Shimadzu) to measure the latter. An inoculation loop was used to spread cells across a diluted casitone agar plate (dCTA) (0·2 % casitone, 0·1 % MgSO4.7H2O, 1·5 % agar). Prey cells were spread in a 4 cm strip starting 1 cm from the edge of the medium. The liquid from prey culture inoculation was allowed to dry prior to inoculating the plates with myxobacteria. Myxobacteria were grown on dCTA and suspended in TPM to a cell density of 0·6 OD600 units. A 20 µl drop of the myxobacterial suspension was placed 0·5 cm from the beginning of the 4 cm strip of prey bacteria. The size of the bacterial strip and the distance lysed was measured after incubation for 14 days at 33 °C. Each experiment, repeated at least once, consisted of three replicates of each M. xanthus strain-and-prey combination. The percentage of the wild-type response calculated was used in statistical analyses of the data. Analysis of variance was conducted on raw or transformed data by separation of means with Tukey's HSD using JMP (Version 4, SAS Institute).
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RESULTS |
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Additionally, the previously described streak assay (Bull et al., 2002) was used to quantify predation by M. xanthus of six different bacterial prey species, including S. marcescens (Table 2
). Based on multiple experiments conducted using this assay, we observed a gradient of susceptibility to predation among the prey species, with E. coli DH5
being the most susceptible, followed by Xanthomonas campestris pv. vitians, Pectobacterium carotovorum subsp. carotovorum, Pseudomonas syringae pv. alisalensis, Sphingomonas suberifaciens, and finally S. marcescens (Table 3
).
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The lawn assay data for the global regulators are similar to the streak assay results when S. marcescens is used as the prey species (Fig. 1b). For the streak assay, predation by the asgA strain is also 27 % of wild-type, and both the csgA and sdeK mutants had about 60 % wild-type predation efficiencies (Fig. 1b
). When the same mutants were assessed for predation ability of five other prey species, the results revealed prey-specific effects (Fig. 1b
). Whereas the asgA mutant suffered defects of between 27 and 74 % on five of the prey species analysed (Pseud. syringae had a minimal defect of about 90 % compared to wild-type), the csgA and sdeK mutants showed significant defects only for predation on Pseud. syringae, S. marcescens, and Sphing. suberifaciens. However, while csgA and sdeK mutants showed quantitatively similar defects when tested on S. marcescens and Sphing. suberifaciens (about 6070 % of wild-type), predation of Pseud. syringae was affected the most by the csgA mutation (about 30 % of wild-type), while the defect due to the sdeK mutation was relatively mild (about 80 % of wild-type).
Several Asg genes are required for wild-type predation
Based on the observed defect of the asgA mutant in our predation assays, we were interested in determining to what extent A signalling may be involved in predation. To this end, the predation phenotypes of mutants in three additional Asg genes asgB480 (Kuspa & Kaiser, 1989), asgC767 (Kuspa & Kaiser, 1989
) and asgE : :
5003 (Garza et al., 2000
) were determined. Like the asgA mutant, mutations in asgC and asgE caused a defect in predation (Table 4
). However, the asgB mutant showed no predation defect, which suggests that the A signal is not necessary for predation and that some other function of the AsgACE regulon plays an important role in regulating the predatory process.
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In contrast, the decreased swarm sizes observed for S-motility mutants can be wholly attributed to their decreased motility on the substrate (Fig. 2). This was true for mutations causing abnormal assembly of both type IV pili and fibrils, two different extracellular components that are essential for S motility (for a review, consult Spormann, 1999
). These results showed that, by our assay, S mutations by themselves do not cause a predation phenotype. However, because S motility is more effective on soft agar (Shi & Zusman, 1993
), our assay cannot entirely rule out that S motility plays a predation-dependent role on soft and wet surfaces. What the results do show, though, is that when both the A- and S-motility systems are knocked out, as in strain DK1250, M. xanthus can neither move nor prey on S. marcescens. This defect was even more severe than those for the A mutants. Thus, even if the S-motility system may not be essential for predation by itself, M. xanthus swarms cannot expand and utilize S. marcescens as a food source without at least one functioning motility system.
Frz mutants are defective in predation
To complete our studies on the role of motility in predation, we determined the predation phenotype of several Frz mutants by the lawn assay. In M. xanthus, the Frz proteins control the reversal frequency for cell movement and are homologous to the Che proteins of E. coli (Blackhart & Zusman, 1985; McBride et al., 1989
; McCleary & Zusman, 1990
). As a result, it has been proposed that the Frz system is involved in modulating directed cell movements in M. xanthus (Kearns & Shimkets, 1998
; Shi et al., 1993
), and thus could play an important role in predation. Mutants in five frz genes were assayed, along with the respective wild-type parent (Table 4
). In all cases, predation was decreased by between 62 and 85 % of wild-type, which strongly suggests a role for tactic behaviour in predation. Previous studies demonstrated that frz mutants, like S-motility mutants, have a defect in swarming on soft and wet surfaces, such as 0·3 % agar, and only a mild defect on firm surfaces, such as 1·5 % agar (Bustamante et al., 2004
). When tested on nitrocellulose in the absence of prey, swarm sizes for frz mutants were found to differ only to a negligible extent (10 % or less) from the wild-type control (data not shown). Therefore, the observed predation defects of the frz strains on the firm surface of the lawn assay are due to a defect in tactic functions rather than in motility.
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DISCUSSION |
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Conceptually, predation can be thought of as a behaviour that, like development, is dependent on a series of interlinked processes required for myxobacterial nutrient scavenging. Among these are systems for hunger alert (signalling), predator-to-predator communication (quorum sensing), prey detection (chemotaxis or chemokinesis), prey tracking (motility), production of killing implements (biolytic compounds) and feeding (lysis of prey and nutrient absorption). While some of these processes have been studied individually, the overall contribution of relevant genetic components to predation has received scant attention in the past.
Here, we demonstrated the involvement of some of these processes in predation by assessing the predation phenotypes of relevant mutants. We developed a simple assay to quantify predation by M. xanthus, which is easily the most well-characterized myxobacterium. The assay measures predation under conditions in which the consumption of live prey is essential for maintaining growth. This is in contrast to the previously reported streak assay, which measures predation of prey in the presence of limited biotic nutrients. Under the conditions of the streak assay, variations in sensitivity to predation by M. xanthus were detected among different prey bacteria (Table 3). The fact that S. marcescens was the species least sensitive to lysis in the streak assay suggested that using S. marcescens to score predation ability in the lawn assay would give the fewest false positive results. However, it is equally possible that the gradient of prey susceptibility observed is specific for the conditions of the streak assay, perhaps because the presence of biotic nutrients limits predation of certain prey bacteria. Regardless of this, the crucial point is that two distinct assays, measuring predation under different conditions, are available to monitor and quantify changes in the predation ability of a variety of M. xanthus strains. Since predation is complex and results may vary depending on the method used, both assays were employed in the study reported here. Strikingly, for those mutants for which data were gathered by both methods, the results were very similar. This implies that relevant pathways important for predation may be turned on in M. xanthus whenever prey is available, regardless of whether alternative sources of sustenance (e.g. in the form of limited biotic nutrients) are present.
Predation and development
The most severe predation defect was observed for strains with mutations in three known A-signal genes, asgA, asgC and asgE (Fig. 1, Table 4
). AsgA is a signal transduction protein required for generating the quorum-sensing A signal, which informs starving M. xanthus cells of whether the colony has reached a threshold cell density to initiate development (Kuspa et al., 1992
). Mutations in asgA cause decreased A-signal production and aborted development (Kuspa & Kaiser, 1989
). A defect in AsgC, a homologue of the vegetative sigma factor RpoD that is predicted to have a variety of effects on gene expression (Davis et al., 1995
; Hernandez & Cashel, 1995
), is similarly defective for A-signal production and development (Kuspa & Kaiser, 1989
). AsgE, which is homologous to amidohydrolases that cleave heterocyclic compounds (Garza et al., 2000
), is believed to function downstream of AsgA: while extracts from asgE cells can partly rescue the asgA sporulation defect, the reciprocal rescue of asgE cells by asgA cells does not occur.
Unlike the asgA, asgC and asgE strains examined, a strain defective for AsgB, which is required for generating at least two extracellular proteases that have been implicated in generating the A signal (Plamann et al., 1992), behaved like wild-type in our assay. This implies that the A signal itself is not required for predation, but that other genes under the control of AsgA and AsgC, such as asgE, are involved in predatory behaviours. These results support our previous hypothesis that A signalling is complex and may represent a series of signals and proteins under the control of AsgA (Garza et al., 2000
).
Somewhat less pronounced, though consistently detected, predation phenotypes were also detected for csgA and sdeK mutants (Fig. 1a, b). The defects, at around 6070 % of wild-type levels, were almost identical for these two regulators (within experimental error). CsgA and SdeK are active at the same time, and function in parallel pathways that converge on common targets (Pollack & Singer, 2001
). However, mutations in devR and 4406, components that act downstream of both pathways (Kroos et al., 1986
), did not exhibit measurable defects (Fig. 1a
). One interpretation is that the predation function of the CsgA and SdeK pathways is mediated through other components not yet identified. We predict that such hypothetical components are likely turned on soon after SdeK and CsgA become active, since it appears that the earlier a regulator functions in development, the more severe the predation defect. This suggests a model whereby early developmental components regulate processes that are also important for predation of S. marcescens, such as detecting nutrient availability, while later components function in pathways that are specific for development. Consistent with this model is the observation that PhoP4 is also not important for predation (Fig. 1a
), although it is an essential regulator of developmental phosphatases that are activated only after 24 h into development (V. D. Pham and others, unpublished results).
The above model is based on data collected for predation of S. marcescens. When M. xanthus predation of other prey bacteria was tested using the streak assay, prey-specific responses among the genetic mutants became evident (Fig. 1b, Table 3
) which suggests a high degree of complexity for the genetic circuitries that regulate predation behaviour. Perhaps predation of different prey species is effected through different pathways, each of which could regulate specialized predation mechanisms mediated by differential production of antimicrobials or by hydrolytic enzymes that would affect potential prey species differently. Possessing multiple predation pathways would allow M. xanthus to make specific, tailored responses depending on the availability of different subsets of prey species in the soil.
Predation and motility
Most methods for isolating bacteriolytic myxobacteria depend on the movement of myxobacterial cells from the originating inoculation material usually a patch of soil or a dung pellet onto an agar substrate containing prey bacteria (for a review, consult Dawid, 2000). This observation suggests an important role for motility in predation. Here, we directly examined the role of the A- and S-motility systems, as well as the Frz chemotaxis-like system, by assaying relevant mutants for predatory ability.
First, we tested the non-motile strain DK1250 (aglB1 tgl-1), which has a mutation in both the A- and S-motility systems (Hodgkin & Kaiser, 1979a). Consistent with the prediction that motility is an important predation factor, these A S cells were unable to prey on feeder bacteria (Fig. 2
). Among the S-motility mutants examined, all mutants displayed significantly reduced swarm sizes; however, these defects were very similar to those attributed solely to the reduced mobility of these mutants on nitrocellulose in the absence of prey (Fig. 2
). Thus, the conclusion is that, under the conditions of our assay, S motility is not important for predation.
In contrast, the A-motility system does appear to be essential for predation. The two A mutants examined had defects in predation of between 35 and 55 % after motility-associated effects were factored out. The most well-characterized A-motility gene is cglB, which appears to code for an outer-membrane lipoprotein important for regulating reversal frequency (Rodriguez & Spormann, 1999; Spormann & Kaiser, 1999
). Interestingly, the cglB mutant is able to secrete the polyelectrolyte gel thought to power A motility, though its uniform deposition around the cell rather than exclusively from the poles suggests an anatomical defect to which its A phenotype can be attributed (Wolgemuth et al., 2002
).
The A-motility system powers movement of individual cells along slime trails, and it is tempting to hypothesize that the observed defect in predation could be ascribed to an inability to track chemotactic gradients laid down by prey bacteria. Our observation that the Frz system is required for predation supports this hypothesis. Different investigators working on various myxobacterial species, including M. xanthus, recorded biased movement towards food bacteria (Daft & Stewart, 1973; Dworkin, 1983
; Kühlwein & Reichenbach, 1968
). However, chemotaxis during vegetative growth is not well-characterized in M. xanthus, and models have been advanced suggesting that M. xanthus gliding motility is too slow to allow for chemotaxis of small, fast-diffusing molecules, though chemokinesis of such molecules may be accommodated (Dworkin & Eide, 1983
; Shi & Zusman, 1994
). Recently, data were presented for the directed movement of M. xanthus cells in response to fatty acid chemoattractants (Kearns et al., 2001
), which suggests that these cells are able to track large molecules with slow diffusion rates. Since the detection of prey or prey trace signatures may be an integral part of predatory behaviour, much could be learned about chemotaxis in M. xanthus by assessing the predation abilities of relevant genetic mutants in greater detail. It has been reported that there are eight clusters of genes encoding Che-like components in M. xanthus (The Institute for Genomic Research). Here, we reported the importance of one of these systems, the Frz system, to predation, but future studies will be needed to determine if additional chemotaxis-like systems also have a role to play.
Concluding remarks
The data presented here demonstrate that predation in M. xanthus is dependent upon genes required for predation-associated processes, such as signalling and motility. However, the components involved may not play uniformly significant roles for predation of all susceptible prey species, and many more additional predation factors almost certainly remain to be identified. It is likely that these will include strain-specific biolytic factors useful for predation of different subsets of prey, as well as conserved global regulators instructing predation pathways common to all strains. The discovery of all these components is a prerequisite for a more thorough understanding of the genetics of M. xanthus predation.
Although bacterial predation by myxobacteria has been demonstrated in soil microcosms (Casida, 1980; Liu & Casida, 1983
), little fundamental information about factors influencing predation in nature is available. In this manuscript, we have described the interaction between myxobacterial predators in the form of developmental mutants and bacterial prey strains, thus providing the first information about developmental factors that influence predation. Although we cannot directly extrapolate these results to natural systems, they do provide us with interesting hypotheses to test in soil systems. Therefore, we have begun to evaluate the role of the mutants with the most pronounced phenotypes in assays evaluating inhibition and predation of plant pathogens in soil systems. For example, our preliminary data indicate that developmental mutants can reduce the control of diseases caused by fungal pathogens (C. T. Bull, unpublished data). In order to clearly describe predation and antagonism in terrestrial ecosystems, we are developing additional tools, including culture-independent enumeration techniques (Pepper et al., 2004
), to study myxobacterial predation and antibiosis in soil systems.
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ACKNOWLEDGEMENTS |
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Received 9 December 2004;
revised 17 February 2005;
accepted 17 February 2005.
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