However, in most cases antimicrobial metabolites are produced by antagonists directly on the spot where the targeted organism is harmful.

Risks assessments for MBCAs are relevant if they contain antimicrobial metabolites at effective concentration in the product. Screening of MBCAs in bioassays on plants or plant tissues has the advantage that MBCAs with multiple modes of action and their combinations potentially can be detected whereas simplified assays on nutrient media strongly bias the selection toward in vitro production of antimicrobial metabolites which may not be responsible for in situ antagonism. Preferences for certain modes of action for an envisaged application of a MBCA also have impact on the screening methods used to select new microbials. Also understanding the mode of action is important to be able to characterize possible risks for humans or the environment and risks for resistance development against the MBCA. Understanding the mode of action of MBCAs is essential to achieve optimum disease control. The potential of microorganisms to produce such a compound in vitro does not necessarily correlate with their in situ antagonism. Compounds involved such as signaling compounds, enzymes and other interfering metabolites are produced in situ at low concentrations during interaction. Such interactions are highly regulated cascades of metabolic events, often combining different modes of action.

Antagonists acting through hyperparasitism and antibiosis are directly interfering with the pathogen. Other MBCAs act via nutrient competition or other mechanisms modulating the growth conditions for the pathogen. Some MBCAs interact with plants by inducing resistance or priming plants without any direct interaction with the targeted pathogen. Microbial biological control agents (MBCAs) are applied to crops for biological control of plant pathogens where they act via a range of modes of action.

Microbes as biocontrol agents