Proteomic Adaptation of Streptococcus pneumoniae to the Human Antimicrobial Peptide LL-37

Abstract

Secreted antimicrobial peptides (AMPs) are an important part of the human innate immune system and prevent local and systemic infections by inhibiting bacterial growth in a concentration-dependent manner. In the respiratory tract, the cationic peptide LL-37 is one of the most abundant AMPs and capable of building pore complexes in usually negatively charged bacterial membranes, leading to the destruction of bacteria. However, the adaptation mechanisms of several pathogens to LL-37 are already described and are known to weaken the antimicrobial effect of the AMP, for instance, by repulsion, export or degradation of the peptide. This study examines proteome-wide changes in Streptococcus pneumoniae D39, the leading cause of bacterial pneumonia, in response to physiological concentrations of LL-37 by high-resolution mass spectrometry. Our data indicate that pneumococci may use some of the known adaptation mechanisms to reduce the effect of LL-37 on their physiology, too. Additionally, several proteins seem to be involved in resistance to AMPs which have not been related to this process before, such as the teichoic acid flippase TacF (SPD_1128). Understanding colonization- and infection-relevant adaptations of the pneumococcus to AMPs, especially LL-37, could finally uncover new drug targets to weaken the burden of this widespread pathogen.

Mode-of-Action of Antimicrobial Peptides: Membrane Disruption vs. Intracellular Mechanisms

“…In the unlabeled approach, after co-incubation of bacteria with AMPs, proteins are extracted, digested and fractionated, and the whole proteome is analyzed by mass spectrometry ( Figure 2B ). A recent study used this approach to show the effect of LL-37 peptide on the proteome of Streptococcus pneumoniae D39, and identified alteration in the expression of 105 proteins ( 93 ). Treatment with LL-37 induced upregulation of proteins involved in cell surface modification, including increasing membrane surface charge, and an abundance of ABC transporters.…”

Antimicrobial Peptides: a New Frontier in Antifungal Therapy

“…AMPs represent one of the possible options to overcome the issue of antimicrobial resistance, partly because they are less susceptible than conventional antibiotics to the evolution of resistance from microorganisms. Although some episodes of resistance against AMPs were described ( 24 – 26 ), the “mutant selection window” (MSW), the concentration range in which selective amplification of single-step, drug-resistant mutants can occur, appears to be narrower than for conventional antibiotics ( 27 ). This results in a higher killing rate ( 28 , 29 ) and lower probability of developing resistance ( 30 ).…”

Resistance Mechanisms to Antimicrobial Peptides in Gram-Positive Bacteria

“…Furthermore, mutant strains lacking these genes revealed an increased susceptibility to treatment with LL-37, confirming the employment of multiple defense strategies against AMPs in pneumococci (Majchrzykiewicz et al, 2010). A more recent study evaluating the proteome of pneumococci treated with LL-37 has also reported a large number of proteins with altered abundance, including transporters, proteins involved in gene regulation and cell wall modification, virulence factors (such as Pht family) and the protease HtrA (Mucke et al, 2020). This result suggests that multiple mechanisms cooperate in pneumococcal response to AMPs.…”