Decoding resistance in the age of T6SS warfare

The history of tuberculosis treatment highlights the power of combinatorial approaches in overcoming resistance to antibiotics. In the 1940s, Streptomycin was discovered as the first effective antibiotic against tuberculosis, but resistance soon emerged, prompting the development of multidrug regimens (1). Isoniazid, introduced in the 1950s, and Rifamycins, first used clinically in the 1970s, were combined with Streptomycin to create potent therapies that targeted the pathogen through distinct mechanisms, providing synergistic toxicities and effectively suppressing resistance evolution. Bacteria themselves possess molecular arsenals such as the type VI secretion system (T6SS), which delivers diverse effector toxins, sometimes in combination, to attack competitors (2, 3). Interestingly, de novo resistance to T6SS effectors is not always observed in natural communities, suggesting that bacteria wielding T6SS and multiple effectors may adopt a combinatorial strategy to limit resistance. In PNAS, Smith et al. (4) reveal that bacteria have evolved mechanisms of T6SS attack akin to combinatorial treatment long before humans harnessed these strategies, showcasing nature’s ingenuity in preempting resistance.

The T6SS plays a pivotal role in microbial competition and host–microbe interactions, enabling bacteria to outcompete rivals in densely populated environments like soil, marine ecosystems, and the gut microbiome by injecting toxic effector proteins directly into neighboring cells (5, 6). This system not only shapes microbial community composition but also influences nutrient cycling and ecosystem dynamics (7–9). Clinically, the T6SS is employed by pathogens such as Pseudomonas aeruginosa and Vibrio cholerae to attack host cells or suppress competing bacteria during infection (10, 11). The ability of targeted microbes to resist or evade T6SS-mediated attacks has significant implications for understanding microbial survival strategies and developing interventions to manage infections and microbial communities.

The T6SS is a molecular spear that many bacteria use to inject toxic proteins directly into neighboring cells through cell–cell contact, distinguishing it from diffusible antibiotics that spread passively through the environment (12). This direct delivery enables precise targeting in crowded microbial communities where diffusion-based approaches are less effective. Previous studies have shown that microbes can evolve spontaneous resistance to T6SS attacks (13, 14) or develop pan resistance by, for example, overproducing exopolysaccharides, which form a protective shield against these assaults (15). Despite the existence of these resistance strategies, it is surprising that bacteria deploying multiple T6SS effectors do not promote the evolution of these or other broad resistance mechanisms in natural communities (16). This suggests that attackers could employ multitoxin approaches to circumvent resistance, highlighting the complexity of bacterial competitive interactions.

Using a combination of agent-based modeling and evolutionary experiments, Smith et al. (4) demonstrate that T6SS multitoxin attacks can suppress resistance evolution (Fig. 1). Multitoxin attacks suppressed resistance via two mechanisms: by limiting the range of mutations subject to positive selection and by driving population extinctions more frequently thus preventing adaptation. The authors also found that T6SS attackers armed with a single-toxin (Tae1) were more likely to induce resistance to multiple toxins than attackers utilizing multitoxin strategies. This finding was unexpected and showed that resistance to one toxin can also provide cross-protection against others, potentially as a result of genetic linkage (one mutation confers resistance to multiple toxins) and/or the accumulation of multiple mutations through drift.

Fig. 1.

Multitoxin attackers limit resistance evolution in susceptible competitors. Attacking bacteria (Top, green) use T6SSs (gray) to secrete single toxins (yellow or red pacman icons), which ultimately result in high levels of resistance in susceptible bacteria (orange) through resistance evolution (yellow or red shield icons). T6SS attacks with multiple toxins suppress resistance evolution by preventing adaptation. Single-toxin attackers can also drive cross-resistance to unfamiliar toxins.

In PNAS, Smith et al. reveal that bacteria have evolved mechanisms of T6SS attack akin to combinatorial treatment long before humans harnessed these strategies, showcasing nature’s ingenuity in preempting resistance.

The discovery that microbes using T6SS multitoxin attacks mitigate the evolution of toxin resistance offers exciting opportunities to advance both antimicrobial and therapeutic strategies. Understanding how bacteria coordinate the deployment of multiple effectors to overcome resistance could inform the design of novel approaches to kill clinically relevant pathogens, such as P. aeruginosa and Klebsiella pneumoniae, both of which can be susceptible to T6SS attacks during infections (10, 17). These insights could guide the development of engineered microbial systems or synthetic antimicrobial platforms that replicate the synergistic efficacy of T6SS attacks (4, 18). Additionally, this discovery provides a model for optimizing combination therapies in other contexts, such as cancer treatment or antiviral therapies, where resistance evolution is a significant challenge. Lessons from the precision and coordination of T6SS effectors could improve drug delivery strategies and help fine-tune combinations of agents with distinct modes of action to enhance treatment outcomes (12, 19). The work by Smith et al. reveals the surprising efficiency of multitoxin T6SS attacks in preventing resistance.