Microbial architects of cigar fermentation: a critical review of beneficial roles in quality enhancement and detrimental potential for mould spoilage.

Cigar tobacco fermentation is a microbially driven process that transforms raw tobacco leaves into a product with distinctive sensory attributes, yet the current understanding of the microbial roles in this process remains fragmented between descriptive community surveys and isolated mechanistic studies, with beneficial and detrimental microbial functions rarely integrated into a unified risk-benefit assessment. This review critically examines the microbiology of cigar fermentation through a dual-axis framework organized around beneficial metabolic functions and detrimental spoilage potential, each resolved into microbial identity, biochemical mechanism, and environmental modulation dimensions. We synthesize evidence from culture-dependent and culture-independent studies on microbial community assembly and succession, where Bacillus, Staphylococcus, and Aspergillus emerge as core fermentation genera, and evaluate the complementary three-pathway system - macromolecular enzymatic degradation, targeted biotransformation of tobacco alkaloids and polyphenols, and de novo biosynthesis of aroma-active volatiles - that drives flavor and quality enhancement. We further analyze the contrastive microbial balance governing tobacco-specific nitrosamine (TSNA) formation, where nitrate-reducing bacteria compete with nitrate-assimilating and nitrite-scavenging microorganisms to determine the net TSNA load. In parallel, we critically examine the mould spoilage microbiology of cigar fermentation, identifying the environmental thresholds - humidity above 80% RH, water activity above 0.85, and inadequate aeration - that select for mycotoxigenic Aspergillus and Penicillium species producing aflatoxins and ochratoxin A at levels that persist into the finished product. We survey emerging biotechnological strategies spanning bioaugmentation with defined starter cultures, biostimulation through environmental optimization, and biocontrol of spoilage fungi, and identify five critical research gaps - including the absence of gnotobiotic fermentation models and the predominance of correlative over causal studies - that must be addressed to translate microbial ecology into predictable fermentation biotechnology. By integrating microbial ecology, fermentation biochemistry, spoilage prevention, and applied biotechnology, this review is intended for researchers in tobacco microbiology and fermentation science, as well as cigar manufacturers, quality-control practitioners, and biotechnologists seeking microbiome-based strategies for quality improvement and risk mitigation.