Murray JL, Kwon T, Marcotte EM, Whiteley M. Intrinsic Antimicrobial Resistance Determinants in the Superbug Pseudomonas aeruginosa. MBio. 2015;6 (6).Abstract
UNLABELLED: Antimicrobial-resistant bacteria pose a serious threat in the clinic. This is particularly true for opportunistic pathogens that possess high intrinsic resistance. Though many studies have focused on understanding the acquisition of bacterial resistance upon exposure to antimicrobials, the mechanisms controlling intrinsic resistance are not well understood. In this study, we subjected the model opportunistic superbug Pseudomonas aeruginosa to 14 antimicrobials under highly controlled conditions and assessed its response using expression- and fitness-based genomic approaches. Our results reveal that gene expression changes and mutant fitness in response to sub-MIC antimicrobials do not correlate on a genomewide scale, indicating that gene expression is not a good predictor of fitness determinants. In general, fewer fitness determinants were identified for antiseptics and disinfectants than for antibiotics. Analysis of gene expression and fitness data together allowed the prediction of antagonistic interactions between antimicrobials and insight into the molecular mechanisms controlling these interactions. IMPORTANCE: Infections involving multidrug-resistant pathogens are difficult to treat because the therapeutic options are limited. These infections impose a significant financial burden on infected patients and on health care systems. Despite years of antimicrobial resistance research, we lack a comprehensive understanding of the intrinsic mechanisms controlling antimicrobial resistance. This work uses two fine-scale genomic approaches to identify genetic loci important for antimicrobial resistance of the opportunistic pathogen Pseudomonas aeruginosa. Our results reveal that antibiotics have more resistance determinants than antiseptics/disinfectants and that gene expression upon exposure to antimicrobials is not a good predictor of these resistance determinants. In addition, we show that when used together, genomewide gene expression and fitness profiling can provide mechanistic insights into multidrug resistance mechanisms.
Palmer GC, Whiteley M. Metabolism and Pathogenicity of Pseudomonas aeruginosa Infections in the Lungs of Individuals with Cystic Fibrosis. Microbiol Spectr. 2015;3 (4).Abstract
Individuals with the genetic disease cystic fibrosis (CF) accumulate mucus or sputum in their lungs. This sputum is a potent growth substrate for a range of potential pathogens, and the opportunistic bacterium Pseudomonas aeruginosa is generally most difficult of these to eradicate. As a result, P. aeruginosa infections are frequently maintained in the CF lung throughout life, and are the leading cause of death for these individuals. While great effort has been expended to better understand and treat these devastating infections, only recently have researchers begun to rigorously examine the roles played by specific nutrients in CF sputum to cue P. aeruginosa pathogenicity. This chapter summarizes the current state of knowledge regarding how P. aeruginosa metabolism in CF sputum affects initiation and maintenance of these infections. It contains an overview of CF lung disease and the mechanisms of P. aeruginosa pathogenicity. Several model systems used to study these infections are described with emphasis on the challenge of replicating the chronic infections observed in humans with CF. Nutrients present in CF sputum are surveyed, and the impacts of these nutrients on the infection are discussed. The chapter concludes by addressing the future of this line of research including the use of next-generation technologies and the potential for metabolism-based therapeutics.
Estrela S, Whiteley M, Brown SP. The demographic determinants of human microbiome health. Trends Microbiol. 2015;23 (3) :134-41.Abstract
The human microbiome is a vast reservoir of microbial diversity and increasingly recognized to have a fundamental role in human health. In polymicrobial communities, the presence of one species can modulate the demography (i.e., growth and distribution) of other species. These demographic impacts generate feedbacks in multispecies interactions, which can be magnified in spatially structured populations (e.g., host-associated communities). Here, we argue that demographic feedbacks between species are central to microbiome development, shaping whether and how potential metabolic interactions come to be realized between expanding lineages of bacteria. Understanding how demographic feedbacks tune metabolic interactions and in turn shape microbiome structure and function is now a key challenge to our abilities to better manage microbiome health.
Turner KH, Wessel AK, Palmer GC, Murray JL, Whiteley M. Essential genome of Pseudomonas aeruginosa in cystic fibrosis sputum. Proc Natl Acad Sci U S A. 2015;112 (13) :4110-5.Abstract
Defining the essential genome of bacterial pathogens is central to developing an understanding of the biological processes controlling disease. This has proven elusive for Pseudomonas aeruginosa during chronic infection of the cystic fibrosis (CF) lung. In this paper, using a Monte Carlo simulation-based method to analyze high-throughput transposon sequencing data, we establish the P. aeruginosa essential genome with statistical precision in laboratory media and CF sputum. Reconstruction of the global requirements for growth in CF sputum compared with defined growth conditions shows that the latter requires several cofactors including biotin, riboflavin, and pantothenate. Comparison of P. aeruginosa strains PAO1 and PA14 demonstrates that essential genes are primarily restricted to the core genome; however, some orthologous genes in these strains exhibit differential essentiality. These results indicate that genes with similar molecular functions may have distinct genetic roles in different P. aeruginosa strains during growth in CF sputum. We also show that growth in a defined growth medium developed to mimic CF sputum yielded virtually identical fitness requirements to CF sputum, providing support for this medium as a relevant in vitro model for CF microbiology studies.
Barnabie PM, Whiteley M. Iron-Mediated Control of Pseudomonas aeruginosa-Staphylococcus aureus Interactions in the Cystic Fibrosis Lung. J Bacteriol. 2015;197 (14) :2250-1.Abstract
Communication is an important factor for bacterial survival, growth, and persistence. Much work has examined both inter- and intraspecies interactions and their effects on virulence. Now, researchers have begun to explore the ways in which host-modulated factors can impact bacterial interactions and subsequently affect patient outcomes. In this issue, two papers discuss how the host environment alters interactions between the pathogens Pseudomonas aeruginosa and Staphylococcus aureus, largely in the context of cystic fibrosis.
Darch SE, Whiteley M. Show me the SNPs. How bacterial sex generates diversity in the cystic fibrosis lung. Am J Respir Crit Care Med. 2015;191 (7) :725-7.
Murray JL, Connell JL, Stacy A, Turner KH, Whiteley M. Mechanisms of synergy in polymicrobial infections. J Microbiol. 2014;52 (3) :188-99.Abstract
Communities of microbes can live almost anywhere and contain many different species. Interactions between members of these communities often determine the state of the habitat in which they live. When these habitats include sites on the human body, these interactions can affect health and disease. Polymicrobial synergy can occur during infection, in which the combined effect of two or more microbes on disease is worse than seen with any of the individuals alone. Powerful genomic methods are increasingly used to study microbial communities, including metagenomics to reveal the members and genetic content of a community and metatranscriptomics to describe the activities of community members. Recent efforts focused toward a mechanistic understanding of these interactions have led to a better appreciation of the precise bases of polymicrobial synergy in communities containing bacteria, eukaryotic microbes, and/or viruses. These studies have benefited from advances in the development of in vivo models of polymicrobial infection and modern techniques to profile the spatial and chemical bases of intermicrobial communication. This review describes the breadth of mechanisms microbes use to interact in ways that impact pathogenesis and techniques to study polymicrobial communities.
Jorth P, Turner KH, Gumus P, Nizam N, Buduneli N, Whiteley M. Metatranscriptomics of the human oral microbiome during health and disease. MBio. 2014;5 (2) :e01012-14.Abstract
The human microbiome plays important roles in health, but when disrupted, these same indigenous microbes can cause disease. The composition of the microbiome changes during the transition from health to disease; however, these changes are often not conserved among patients. Since microbiome-associated diseases like periodontitis cause similar patient symptoms despite interpatient variability in microbial community composition, we hypothesized that human-associated microbial communities undergo conserved changes in metabolism during disease. Here, we used patient-matched healthy and diseased samples to compare gene expression of 160,000 genes in healthy and diseased periodontal communities. We show that health- and disease-associated communities exhibit defined differences in metabolism that are conserved between patients. In contrast, the metabolic gene expression of individual species was highly variable between patients. These results demonstrate that despite high interpatient variability in microbial composition, disease-associated communities display conserved metabolic profiles that are generally accomplished by a patient-specific cohort of microbes. IMPORTANCE The human microbiome project has shown that shifts in our microbiota are associated with many diseases, including obesity, Crohn's disease, diabetes, and periodontitis. While changes in microbial populations are apparent during these diseases, the species associated with each disease can vary from patient to patient. Taking into account this interpatient variability, we hypothesized that specific microbiota-associated diseases would be marked by conserved microbial community behaviors. Here, we use gene expression analyses of patient-matched healthy and diseased human periodontal plaque to show that microbial communities have highly conserved metabolic gene expression profiles, whereas individual species within the community do not. Furthermore, disease-associated communities exhibit conserved changes in metabolic and virulence gene expression.
Wessel AK, Arshad TA, Fitzpatrick M, Connell JL, Bonnecaze RT, Shear JB, Whiteley M. Oxygen limitation within a bacterial aggregate. MBio. 2014;5 (2) :e00992.Abstract
ABSTRACT Cells within biofilms exhibit physiological heterogeneity, in part because of chemical gradients existing within these spatially structured communities. Previous work has examined how chemical gradients develop in large biofilms containing >10(8) cells. However, many bacterial communities in nature are composed of small, densely packed aggregates of cells (≤ 10(5) bacteria). Using a gelatin-based three-dimensional (3D) printing strategy, we confined the bacterium Pseudomonas aeruginosa within picoliter-sized 3D "microtraps" that are permeable to nutrients, waste products, and other bioactive small molecules. We show that as a single bacterium grows into a maximally dense (10(12) cells ml(-1)) clonal population, a localized depletion of oxygen develops when it reaches a critical aggregate size of ~55 pl. Collectively, these data demonstrate that chemical and phenotypic heterogeneity exists on the micrometer scale within small aggregate populations. IMPORTANCE Before developing into large, complex communities, microbes initially cluster into aggregates, and it is unclear if chemical heterogeneity exists in these ubiquitous micrometer-scale aggregates. We chose to examine oxygen availability within an aggregate since oxygen concentration impacts a number of important bacterial processes, including metabolism, social behaviors, virulence, and antibiotic resistance. By determining that oxygen availability can vary within aggregates containing ≤ 10(5) bacteria, we establish that physiological heterogeneity exists within P. aeruginosa aggregates, suggesting that such heterogeneity frequently exists in many naturally occurring small populations.
Kwon T, Huse HK, Vogel C, Whiteley M, Marcotte EM. Protein-to-mRNA ratios are conserved between Pseudomonas aeruginosa strains. J Proteome Res. 2014;13 (5) :2370-80.Abstract
Recent studies have shown that the concentrations of proteins expressed from orthologous genes are often conserved across organisms and to a greater extent than the abundances of the corresponding mRNAs. However, such studies have not distinguished between evolutionary (e.g., sequence divergence) and environmental (e.g., growth condition) effects on the regulation of steady-state protein and mRNA abundances. Here, we systematically investigated the transcriptome and proteome of two closely related Pseudomonas aeruginosa strains, PAO1 and PA14, under identical experimental conditions, thus controlling for environmental effects. For 703 genes observed by both shotgun proteomics and microarray experiments, we found that the protein-to-mRNA ratios are highly correlated between orthologous genes in the two strains to an extent comparable to protein and mRNA abundances. In spite of this high molecular similarity between PAO1 and PA14, we found that several metabolic, virulence, and antibiotic resistance genes are differentially expressed between the two strains, mostly at the protein but not at the mRNA level. Our data demonstrate that the magnitude and direction of the effect of protein abundance regulation occurring after the setting of mRNA levels is conserved between bacterial strains and is important for explaining the discordance between mRNA and protein abundances.
Stacy A, Everett J, Jorth P, Trivedi U, Rumbaugh KP, Whiteley M. Bacterial fight-and-flight responses enhance virulence in a polymicrobial infection. Proc Natl Acad Sci U S A. 2014;111 (21) :7819-24.Abstract
The oral pathogen Aggregatibacter actinomycetemcomitans (Aa) resides in infection sites with many microbes, including commensal streptococci such as Streptococcus gordonii (Sg). During infection, Sg promotes the virulence of Aa by producing its preferred carbon source, l-lactate, a phenomenon referred to as cross-feeding. However, as with many streptococci, Sg also produces high levels of the antimicrobial hydrogen peroxide (H2O2), leading to the question of how Aa deals with this potent antimicrobial during coinfection. Here, we show that Aa possesses two complementary responses to H2O2: a detoxification or fight response mediated by catalase (KatA) and a dispersion or flight response mediated by Dispersin B (DspB), an enzyme that dissolves Aa biofilms. Using a murine abscess infection model, we show that both of these responses are required for Sg to promote Aa virulence. Although the role of KatA is to detoxify H2O2 during coinfection, 3D spatial analysis of mixed infections revealed that DspB is required for Aa to spatially organize itself at an optimal distance (>4 µm) from Sg, which we propose allows cross-feeding but reduces exposure to inhibitory levels of H2O2. In addition, these behaviors benefit not only Aa but also Sg, suggesting that fight and flight stimulate the fitness of the community. These results reveal that an antimicrobial produced by a human commensal bacterium enhances the virulence of a pathogenic bacterium by modulating its spatial location in the infection site.
Sams ER, Whiteley M, Turner KH. 'The battle for life': Pasteur, anthrax, and the first probiotics. J Med Microbiol. 2014;63 (Pt 11) :1573-4.
Kim J, Connell JL, Whiteley M, Bard AJ. Development of a versatile in vitro platform for studying biological systems using micro-3D printing and scanning electrochemical microscopy. Anal Chem. 2014;86 (24) :12327-33.Abstract
We report a novel strategy for studying a broad range of cellular behaviors in real time by combining two powerful analytical techniques, micro-3D printing and scanning electrochemical microscopy (SECM). This allows one, in microbiological studies, to isolate a known number of cells in a micrometer-sized chamber with a roof and walls that are permeable to small molecules and observe metabolic products. In such studies, the size and spatial organization of a population play a crucial role in cellular group behaviors, such as intercellular interactions and communication. Micro-3D printing, a photolithographic method for constructing cross-linked protein microstructures, permits one to compartmentalize a small population of microbes by forming a porous roof and walls around cells in situ. Since the roof and walls defining the microchamber are porous, any small molecules can freely diffuse from the chamber to be detected and quantified using SECM. The size of the chamber and the roof permeability can be obtained by SECM using a small probe molecule, ferrocenemethanol (FcMeOH). The chamber permeability to FcMeOH can be tuned by varying printing parameters that influence the cross-linking density of the proteinaceous material. These analyses establish a versatile strategy as a sensitive platform to quantitatively monitor small molecules produced by microbes.
Hidalgo-Romano B, Gollihar J, Brown SA, Whiteley M, Valenzuela E, Kaplan HB, Wood TK, McLean RJC. Indole inhibition of N-acylated homoserine lactone-mediated quorum signalling is widespread in Gram-negative bacteria. Microbiology. 2014;160 (Pt 11) :2464-73.Abstract
The LuxI/R quorum-sensing system and its associated N-acylated homoserine lactone (AHL) signal is widespread among Gram-negative bacteria. Although inhibition by indole of AHL quorum signalling in Pseudomonas aeruginosa and Acinetobacter oleivorans has been reported previously, it has not been documented among other species. Here, we show that co-culture with wild-type Escherichia coli, but not with E. coli tnaA mutants that lack tryptophanase and as a result do not produce indole, inhibits AHL-regulated pigmentation in Chromobacterium violaceum (violacein), Pseudomonas chlororaphis (phenazine) and Serratia marcescens (prodigiosin). Loss of pigmentation also occurred during pure culture growth of Chro. violaceum, P. chlororaphis and S. marcescens in the presence of physiologically relevant indole concentrations (0.5-1.0 mM). Inhibition of violacein production by indole was counteracted by the addition of the Chro. violaceum cognate autoinducer, N-decanoyl homoserine lactone (C10-HSL), in a dose-dependent manner. The addition of exogenous indole or co-culture with E. coli also affected Chro. violaceum transcription of vioA (violacein pigment production) and chiA (chitinase production), but had no effect on pykF (pyruvate kinase), which is not quorum regulated. Chro. violaceum AHL-regulated elastase and chitinase activity were inhibited by indole, as was motility. Growth of Chro. violaceum was not affected by indole or C10-HSL supplementation. Using a nematode-feeding virulence assay, we observed that survival of Caenorhabditis elegans exposed to Chro. violaceum, P. chlororaphis and S. marcescens was enhanced during indole supplementation. Overall, these studies suggest that indole represents a general inhibitor of AHL-based quorum signalling in Gram-negative bacteria.
Connell JL, Kim J, Shear JB, Bard AJ, Whiteley M. Real-time monitoring of quorum sensing in 3D-printed bacterial aggregates using scanning electrochemical microscopy. Proc Natl Acad Sci U S A. 2014;111 (51) :18255-60.Abstract
Microbes frequently live in nature as small, densely packed aggregates containing ∼10(1)-10(5) cells. These aggregates not only display distinct phenotypes, including resistance to antibiotics, but also, serve as building blocks for larger biofilm communities. Aggregates within these larger communities display nonrandom spatial organization, and recent evidence indicates that this spatial organization is critical for fitness. Studying single aggregates as well as spatially organized aggregates remains challenging because of the technical difficulties associated with manipulating small populations. Micro-3D printing is a lithographic technique capable of creating aggregates in situ by printing protein-based walls around individual cells or small populations. This 3D-printing strategy can organize bacteria in complex arrangements to investigate how spatial and environmental parameters influence social behaviors. Here, we combined micro-3D printing and scanning electrochemical microscopy (SECM) to probe quorum sensing (QS)-mediated communication in the bacterium Pseudomonas aeruginosa. Our results reveal that QS-dependent behaviors are observed within aggregates as small as 500 cells; however, aggregates larger than 2,000 bacteria are required to stimulate QS in neighboring aggregates positioned 8 μm away. These studies provide a powerful system to analyze the impact of spatial organization and aggregate size on microbial behaviors.
Turner KH, Everett J, Trivedi U, Rumbaugh KP, Whiteley M. Requirements for Pseudomonas aeruginosa acute burn and chronic surgical wound infection. PLoS Genet. 2014;10 (7) :e1004518.Abstract
Opportunistic infections caused by Pseudomonas aeruginosa can be acute or chronic. While acute infections often spread rapidly and can cause tissue damage and sepsis with high mortality rates, chronic infections can persist for weeks, months, or years in the face of intensive clinical intervention. Remarkably, this diverse infectious capability is not accompanied by extensive variation in genomic content, suggesting that the genetic capacity to be an acute or a chronic pathogen is present in most P. aeruginosa strains. To investigate the genetic requirements for acute and chronic pathogenesis in P. aeruginosa infections, we combined high-throughput sequencing-mediated transcriptome profiling (RNA-seq) and genome-wide insertion mutant fitness profiling (Tn-seq) to characterize gene expression and fitness determinants in murine models of burn and non-diabetic chronic wound infection. Generally we discovered that expression of a gene in vivo is not correlated with its importance for fitness, with the exception of metabolic genes. By combining metabolic models generated from in vivo gene expression data with mutant fitness profiles, we determined the nutritional requirements for colonization and persistence in these infections. Specifically, we found that long-chain fatty acids represent a major carbon source in both chronic and acute wounds, and P. aeruginosa must biosynthesize purines, several amino acids, and most cofactors during infection. In addition, we determined that P. aeruginosa requires chemotactic flagellar motility for fitness and virulence in acute burn wound infections, but not in non-diabetic chronic wound infections. Our results provide novel insight into the genetic requirements for acute and chronic P. aeruginosa wound infections and demonstrate the power of using both gene expression and fitness profiling for probing bacterial virulence.
Korgaonkar A, Trivedi U, Rumbaugh KP, Whiteley M. Community surveillance enhances Pseudomonas aeruginosa virulence during polymicrobial infection. Proc Natl Acad Sci U S A. 2013;110 (3) :1059-64.Abstract
Most infections result from colonization by more than one microbe. Within such polymicrobial infections, microbes often display synergistic interactions that result in increased disease severity. Although many clinical studies have documented the occurrence of synergy in polymicrobial infections, little is known about the underlying molecular mechanisms. A prominent pathogen in many polymicrobial infections is Pseudomonas aeruginosa, a Gram-negative bacterium that displays enhanced virulence during coculture with Gram-positive bacteria. In this study we discovered that during coinfection, P. aeruginosa uses peptidoglycan shed by Gram-positive bacteria as a cue to stimulate production of multiple extracellular factors that possess lytic activity against prokaryotic and eukaryotic cells. Consequently, P. aeruginosa displays enhanced virulence in a Drosophila model of infection when cocultured with Gram-positive bacteria. Inactivation of a gene (PA0601) required for peptidoglycan sensing mitigated this phenotype. Using Drosophila and murine models of infection, we also show that peptidoglycan sensing results in P. aeruginosa-mediated reduction in the Gram-positive flora in the infection site. Our data suggest that P. aeruginosa has evolved a mechanism to survey the microbial community and respond to Gram-positive produced peptidoglycan through production of antimicrobials and toxins that not only modify the composition of the community but also enhance host killing. Additionally, our results suggest that therapeutic strategies targeting Gram-positive bacteria might be a viable approach for reducing the severity of P. aeruginosa polymicrobial infections.
Needham BD, Carroll SM, Giles DK, Georgiou G, Whiteley M, Trent SM. Modulating the innate immune response by combinatorial engineering of endotoxin. Proc Natl Acad Sci U S A. 2013;110 (4) :1464-9.Abstract
Despite its highly inflammatory nature, LPS is a molecule with remarkable therapeutic potential. Lipid A is a glycolipid that serves as the hydrophobic anchor of LPS and constitutes a potent ligand of the Toll-like receptor (TLR)4/myeloid differentiation factor 2 receptor of the innate immune system. A less toxic mixture of monophosphorylated lipid A species (MPL) recently became the first new Food and Drug Administration-approved adjuvant in over 70 y. Whereas wild-type Escherichia coli LPS provokes strong inflammatory MyD88 (myeloid differentiation primary response gene 88)-mediated TLR4 signaling, MPL preferentially induces less inflammatory TRIF (TIR-domain-containing adaptor-inducing IFN-β)-mediated responses. Here, we developed a system for combinatorial structural diversification of E. coli lipid A, yielding a spectrum of bioactive variants that display distinct TLR4 agonist activities and cytokine induction. Mice immunized with engineered lipid A/antigen emulsions exhibited robust IgG titers, indicating the efficacy of these molecules as adjuvants. This approach demonstrates how combinatorial engineering of lipid A can be exploited to generate a spectrum of immunostimulatory molecules for vaccine and therapeutics development.
Wessel AK, Liew J, Kwon T, Marcotte EM, Whiteley M. Role of Pseudomonas aeruginosa peptidoglycan-associated outer membrane proteins in vesicle formation. J Bacteriol. 2013;195 (2) :213-9.Abstract
Gram-negative bacteria produce outer membrane vesicles (OMVs) that package and deliver proteins, small molecules, and DNA to prokaryotic and eukaryotic cells. The molecular details of OMV biogenesis have not been fully elucidated, but peptidoglycan-associated outer membrane proteins that tether the outer membrane to the underlying peptidoglycan have been shown to be critical for OMV formation in multiple Enterobacteriaceae. In this study, we demonstrate that the peptidoglycan-associated outer membrane proteins OprF and OprI, but not OprL, impact production of OMVs by the opportunistic pathogen Pseudomonas aeruginosa. Interestingly, OprF does not appear to be important for tethering the outer membrane to peptidoglycan but instead impacts OMV formation through modulation of the levels of the Pseudomonas quinolone signal (PQS), a quorum signal previously shown by our laboratory to be critical for OMV formation. Thus, the mechanism by which OprF impacts OMV formation is distinct from that for other peptidoglycan-associated outer membrane proteins, including OprI.
Connell JL, Ritschdorff ET, Whiteley M, Shear JB. 3D printing of microscopic bacterial communities. Proc Natl Acad Sci U S A. 2013;110 (46) :18380-5.Abstract
Bacteria communicate via short-range physical and chemical signals, interactions known to mediate quorum sensing, sporulation, and other adaptive phenotypes. Although most in vitro studies examine bacterial properties averaged over large populations, the levels of key molecular determinants of bacterial fitness and pathogenicity (e.g., oxygen, quorum-sensing signals) may vary over micrometer scales within small, dense cellular aggregates believed to play key roles in disease transmission. A detailed understanding of how cell-cell interactions contribute to pathogenicity in natural, complex environments will require a new level of control in constructing more relevant cellular models for assessing bacterial phenotypes. Here, we describe a microscopic three-dimensional (3D) printing strategy that enables multiple populations of bacteria to be organized within essentially any 3D geometry, including adjacent, nested, and free-floating colonies. In this laser-based lithographic technique, microscopic containers are formed around selected bacteria suspended in gelatin via focal cross-linking of polypeptide molecules. After excess reagent is removed, trapped bacteria are localized within sealed cavities formed by the cross-linked gelatin, a highly porous material that supports rapid growth of fully enclosed cellular populations and readily transmits numerous biologically active species, including polypeptides, antibiotics, and quorum-sensing signals. Using this approach, we show that a picoliter-volume aggregate of Staphylococcus aureus can display substantial resistance to β-lactam antibiotics by enclosure within a shell composed of Pseudomonas aeruginosa.