Microbial toxins are produced by bacteria that can be toxic and contaminate food crops, especially root vegetables (if found in soil). For example, the bacterium Streptomyces can produce bafilomycin, a toxic antibiotic and immune suppressant (Myers et al. 2003). Exposure to these toxins may damage beta cells, aggravate beta cell death and progression to diabetes (Hettiarachchi et al. 2008a).
Also see the Factsheet on Mold from the National Institute of Environmental Health Sciences (NIEHS).
When low levels of balfilomycin is injected into mice, it impairs glucose tolerance, reduces islet cell size, and decreased beta cell mass (Myers et al. 2003). Repeated low level doses of balfilomycin led to lower islet size in female mice. Another microbial toxin, concanamycin, worsened glucose tolerance in female mice (Hettiarachchi et al. 2006a).
When parent non-obese diabetic (NOD) mice (used in the lab as a model for type 1 diabetes), were fed balfilomycin from conception through pregnancy, their offspring had accelerated diabetes development as well as a higher frequency of diabetes. The development of the islet cells in the pancreas was disrupted in the exposed mice (Hettiarachchi et al. 2004).
In other mice, in utero exposure to balfilomycin disrupts the development of beta cells and encourages beta cell death (Hettiarachchi et al. 2008b).
Beta cells treated with balfilomycin alters insulin signalling in beta cells. Low levels increase insulin signalling, and higher levels lead to beta cell death. Toxins like this that interfere with insulin signalling could promote diabetes development by interfering with the growth of beta cells (Hettiarachchi et al. 2006b).
Cereulide, a toxin produced by a bacteria linked to food poisonings, is toxic to beta cells (Vangoitsenhoven et al. 2015; Virtanen et al. 2008). At low levels, it causes beta cell death, and at even lower levels, impairs beta cell function (Vangoitsenhoven et al. 2014).
Microcystin-LR, produced by freshwater bacteria and algal blooms (and which can be in drinking water), also is toxic to beta cells (Zhao et al. 2017; Zhao et al. 2016).
Based on the above studies, eating root crops such as potatoes during pregnancy might be expected to increase the risk of type 1 diabetes in offspring later in life. However, potato consumption during pregnancy has been associated with a delayed later risk of type 1-related autoimmunity in children (Lamb et al. 2008).
According to a review, "a final conclusion concerning the causal role of microbes in the pathogenesis of T1DM has not been made." (Lammi et al. 2005).
To see studies on microbial toxins and diabetes, see this collection in PubMed: Microbial toxins and diabetes/obesity.