Recently, the drawbacks arising from the overuse of antibiotics have drawn growing public attention. Among them, drug-resistance (DR) and even multidrug-resistance (MDR) pose significant challenges in clinical practice. As a representative of a DR or MDR pathogen, Staphylococcus aureus can cause diversity of infections related to different organs, and can survive or adapt to the diverse hostile environments by switching into other phenotypes, including biofilm and small colony variants (SCVs), with altered physiologic or metabolic characteristics. In this review, we briefly describe the development of the DR/MDR as well as the classical mechanisms (accumulation of the resistant genes). Moreover, we use multidimensional scaling analysis to evaluate the MDR relevant hotspots in the recent published reports. Furthermore, we mainly focus on the possible non-classical resistance mechanisms triggered by the two important alternative phenotypes of the S. aureus, biofilm and SCVs, which are fundamentally caused by the different global regulation of the S. aureus population, such as the main quorum-sensing (QS) and agr system and its coordinated regulated factors, such as the SarA family proteins and the alternative sigma factor σB (SigB). Both the biofilm and the SCVs are able to escape from the host immune response, and resist the therapeutic effects of antibiotics through the physical or the biological barriers, and become less sensitive to some antibiotics by the dormant state with the limited metabolisms.

The increasing numbers of infections caused by multidrug-resistant (MDR) pathogens highlight the urgent need for new alternatives to conventional antibiotics. Antimicrobial peptides have the potential to be promising alternatives to antibiotics because of their effective bactericidal activity and highly selective toxicity. The present study was conducted to investigate the antibacterial, antibiofilm, and anti-adhesion activities of different CTP peptides (CTP: the original hybrid peptide cathelicidin 2 (1-13)-thymopentin (TP5); CTP-NH2: C-terminal amidated derivative of cathelicidin 2 (1-13)-TP5; CTPQ: glutamine added at the C-terminus of cathelicidin 2 (1-13)-TP5) by determining the minimal inhibitory concentrations (MICs), minimal bactericidal concentrations (MBCs), propidium iodide uptake, and analysis by scanning electron microscopy, transmission electron microscopy, and confocal laser scanning microscopy). The results showed that CTPs had broad-spectrum antibacterial activity against different gram-positive and gram-negative bacteria, with MICs against the tested strains varying from 2 to 64 μg/mL. CTPs at the MBC (2 × MIC 64 μg/mL) showed strong bactericidal effects on a standard methicillin-resistant Staphylococcus aureus strain ATCC 43300 after co-incubation for 6 h through disruption of the bacterial membrane. In addition, CTPs at 2 × MIC also displayed effective inhibition activity of several S. aureus strains with a 40–90% decrease in biofilm formation by killing the bacteria embedded in the biofilms. CTPs had low cytotoxicity on the intestinal porcine epithelial cell line (IPEC-J2) and could significantly decrease the rate of adhesion of S. aureus ATCC 43300 on IPEC-J2 cells. The current study proved that CTPs have effective antibacterial, antibiofilm, and anti-adhesion activities. Overall, this study contributes to our understanding of the possible antibacterial and antibiofilm mechanisms of CTPs, which might be an effective anti-MDR drug candidate.

Immunity is a versatile defensive response that is involved in protecting against disease by identifying and destroying self and non-self harmful substances. As a state of temporary or permanent immune dysfunction, immunosuppression can make an organism more susceptible to infection, organ injury, and cancer due to damage to the immune system. It has taken a long time to develop new immunomodulatory agents to prevent and treat immunosuppressive diseases. In recent years, Toll-like receptor 2 (TLR2) agonists have been reported to have profound effects on the immune system, and they are regarded as potent immunomodulatory candidates. TP5 and LL-37, the potent immunomodulatory agents, have been reported to produce a robust innate immune response by binding to TLR2. However, their development has been weakened by several concerns, such as potential cytotoxicity, weak physiological stability and poor immunomodulatory activity. To overcome these challenges, hybridization has been proposed. Therefore, six hybrid peptides (LTPa, LTPb, LTPc, TPLa, TPLb, and TPLc) were designed by combining the full-length TP5 with a characteristic fragment of LL-37 that included LL-37 (13–36), LL-37 (17–29), and LL-37 (13–31). LTPa, the most potent TLR2 agonist, was simply and effectively screened by molecular docking and in vitro experiments. Furthermore, the immunomodulatory effects of LTPa were confirmed by a CTX-immunosuppressed murine model, which demonstrated that LTPa successfully inhibit immunosuppression, increased immune organ indices, enhanced DC maturation, regulated T lymphocyte subsets, and increased cytokine and Ig contents. Our study also revealed that the immunomodulatory effects of LTPa are associated with binding to TLR2, forming TLR2 clusters, and activating the NF-κB signaling pathway.

In this study, the yield of exopolysaccharide (EPS) from Lactobacillus plantarum R301 was optimized using a single-factor experiment and response surface methodology (RSM). After optimization, the EPS yield was increased with a fold-change of 0.85. The significant factors affecting EPS production, as determined through a Plackett–Burman design and Central Composite Design (CCD), were MgSO4 concentration, initial pH, and inoculation size. The maximum yield was 97.85 mg/mL under the condition of 0.01% MgSO4, an initial pH 7.4, and 6.4% of the inoculation size. In addition, the EPS exhibited strong antioxidant activity, as demonstrated by its ability to scavenge DPPH, ABTS, and hydroxyl radicals. The scavenging rate was up to 100% at concentrations of 4 mg/mL, 1 mg/mL, and 2 mg/mL, respectively. Moreover, the EPS also exhibited reducing power, which was about 30% that of ascorbic acid when both tended to be stable with the increased concentration. These results suggest that L. plantarum R301 EPS possesses different antioxidant mechanisms and warrants further investigation. In addition to its antioxidant activity, the EPS also demonstrated good anti-inflammatory activity by inhibiting the inflammation induced by lipopolysaccharide (LPS) in RAW 264.7 cells, which could decrease nitric oxide (NO) production and expression of the proinflammatory cytokine Il-6. These findings suggest that L. plantarum R301 EPS could be used as a potential multifunctional food additive in the food industry.

The current study aimed to investigate the effects of Clostridium butyiricum on growth performance, intestinal morphology, serum biochemical response, and immunity in broiler chickens. A total of 330 commercial one-day-old, mixed-sex Ross 308 broilers were randomly divided into five treatment groups with six replicates per group. The broilers were fed the basal diet (CON), the basal diet with 150 mg/kg of aureomycin (AM), the basal diet with C. butyricum at 2 × 108 CFU/kg (CBL), the basal diet with C. butyricum at 4 × 108 CFU/kg (CBM), and the basal diet with C. butyricum at 8 × 108 CFU/kg (CBH). Results showed that the final body weight (BW) (p < 0.01; p < 0.05), ADG from day 22 to 39 (p < 0.05), and ADG from day 1 to 39 (p < 0.01; p < 0.05) were improved in a linear and quadratic response with the inclusion of C. butyricum. There were no differences in feed conversion rate (FCR) among all groups (p > 0.05). Supplementation with C. butyricum quadratically reduced the crypt depth at day 21 (p < 0.01), linearly improved the villus height in the jejunum at day 39 (p < 0.001), and linearly and quadratically increased the villus height to crypt depth (V/C) ratio in the jejunum at day 21 (p < 0.01) and day 39 (p < 0.01; p < 0.001). Dietary C. butyricum affected the thymus index at day 21 and day 39 (linear, p < 0.01), and the bursa of Fabricius index at day 39 (quadratic, p < 0.05). Compared to the AM group, the serum urea contents were decreased (p < 0.05) but the IgG contents were increased in the CBL and CBH groups at day 21 (p < 0.01); in addition, serum albumin (ALB) concentrations in all the C. butyricum-supplemented groups (p < 0.01) and IgG concentrations in the CBM group were augmented at day 39 (p < 0.05). In conclusion, dietary C. butyricum could enhance growth performance by improving jejunal morphology and stimulating immunity organ development in broilers, and could be an alternative to antibiotics in poultry feeds.