The biological basis of severe bacterial infection unveiling from heterogeneity induction

Severe invasive bacterial infections are among the most difficult bacterial infections to treat because of their rapid symptom progression and high mortality rate. Severe invasive Streptococcal infections, in particular, develop suddenly inpatients with little or no underlying serious illness, such as immunodeficiency. Initial symptoms include pain, swelling, fever, and hypotension in the extremities, but there are no characteristic symptoms. Within 24 hours from symptom, the disease causes soft tissue necrosis, acute renal failure, adult-onset respiratory distress syndrome, disseminated intravascular coagulation syndrome, and multiple organ failure, leading to death from shock symptoms, with a fatality rate of 30%-40%. Because Group A Streptococcus and Group C/G Streptococcus are the most common causative organisms, penicillin and clindamycin are recommended for treatment. However, this invasive infection often causes circulatory failure and inadequate response to antimicrobial agents, and soft tissue necrosis in the extremities necessitates procedures such as debridement and amputation of the necrotic area. In many cases, soft-tissue necrosis of the extremities also leads to inadequate response to antimicrobial agents, and poor prognosis. Unfortunately, despite the urgent need, no effective diagnostic or therapeutic methods have been developed yet. Whole-genome analysis of "invasive strains" has been conducted vigorously by researchers worldwide; although characteristics, such as a tendency to have many covR/S (master regulator of pathogenic genes) mutants, were found, no specific strains were observed to be prevalent, and no invasive-strain-specific pathogenetic factors were identified. In this context, we showed that the isolates from individual clinical Streptococcus pyogenes samples are not a monoclonal population but rather are a heterogeneous population in which multiple strains with mutated genomes exist simultaneously and that there is a high possibility that the causative strains for invasive infection are among them. When we attempted to reproduce this heterogeneity induction in our cell culture system for autophagy, we found that autophagy may lead to the emergence of bacteria with a heterogeneous invasive phenotype with rapidly induced genomic mutations. Interestingly, the intracellular invasive mutants were derived from the survivors of autophagic degradation, i.e., when mutation was accelerated by some stimulation of a low-pathogenic strain by autophagy, strains that lead to invasive infection could emerge. Such a "rapid virulence transformation of bacteria in host cells" had never been reported, and that work revealed an entirely new mechanism of virulence acquisition by pathogenic bacteria. Therefore, to control the onset of invasive infections, knowledge of invasive infection characteristics and invasive disease pathogenesis must be applied to the development of antimicrobial agents or drugs directed against invasive strains, such as low-molecular-weight compounds and fragment antibodies that inhibit disease progression (e.g., antibodies that neutralize secreted proteins or inhibitors of their functions).