Chlorhexidine digluconate (20% in water)

Infectious-disease experiments test how pathogens interact with host barriers, innate immune receptors, inflammatory signaling, pathogen replication, and tissue injury; pattern-recognition receptors such as TLRs, RIG-I-like receptors, NOD-like receptors, and inflammasomes detect microbial molecules and activate NF-κB, interferon, and cytokine responses. The central hypothesis is that infection severity reflects the balance between pathogen burden and host response: protective inflammation restricts pathogen growth, whereas excessive or mislocalized inflammation contributes to tissue damage and disease phenotype. Unresolved questions include which host pathways are protective versus pathogenic, why some infection models fail to translate to human disease, and which combined readouts best predict clinically relevant infection outcomes.

Apoptosis, also called programmed cell death, is generally characterized by distinct morphological characteristics.

TUNEL staining detects DNA strand breaks by using terminal deoxynucleotidyl transferase to add labeled nucleotides to exposed 3′-OH DNA termini, generating either microscopic staining in fixed cells or tissue sections, or fluorescence/cytometric signal in cell suspensions. TUNEL positivity reflects DNA fragmentation but should not be interpreted alone as definitive apoptosis, because TUNEL can also label necrotic, autolytic, mechanically damaged, or DNA-repair-associated DNA breaks.

Cytotoxicity assays are usually based on the assessment of cell membrane damage, which can also be indirectly detected by measuring cell viability. Detection methods include MTT assay, CKK-8 assay, LDH assay and ATP assay, etc.

Live/dead viability and cytotoxicity staining assays are based on the simultaneous detection of intracellular esterase activity in metabolically active (viable) cells and membrane integrity loss in non-viable cells. In commonly used dual-staining approaches, membrane-permeant fluorogenic substrates are converted by intracellular esterases into fluorescent products in live cells, while impermeant DNA-binding dyes selectively enter cells with compromised plasma membranes and label nucleic acids in dead or dying cells, enabling discrimination between viable and non-viable populations by fluorescence microscopy or flow cytometry.

Annexin V-based apoptosis assays rely on the detection of phosphatidylserine (PS) externalization from the inner leaflet of the plasma membrane to the outer leaflet, an early biochemical hallmark of apoptosis. Fluorescently labeled Annexin V binds PS in a calcium-dependent manner, enabling identification of early apoptotic cells by flow cytometry or fluorescence microscopy. When combined with a membrane-impermeant DNA-binding dye (e. g. , propidium iodide), this approach allows discrimination between viable (Annexin V−/dye−), early apoptotic (Annexin V+/dye−), and late apoptotic or necrotic (Annexin V+/dye+) cell populations by assessing membrane integrity and PS exposure.

TEER measures electrical resistance across epithelial or endothelial monolayers cultured on permeable supports, and the readout reflects ionic conductance through the cell barrier, especially the paracellular pathway regulated by junctional integrity. TEER can be measured without destroying the monolayer and is commonly used before or during transport, permeability, barrier-disruption, and barrier-maturation experiments. TEER values are influenced by biological maturation and technical conditions; reported factors include temperature, medium formulation, passage number, electrode geometry, membrane properties, and junctional length during early monolayer maturation. Therefore, TEER should be interpreted with blank-insert subtraction, area normalization, repeated readings, and, when possible, orthogonal barrier readouts such as FITC-dextran flux or tight-junction staining.

Apoptosis is a regulated, generally non-lytic cell-death pathway that removes unwanted, damaged, infected, or abnormal cells through coordinated morphological changes, caspase activation, DNA fragmentation, and membrane remodeling. The intrinsic apoptosis pathway is controlled mainly by mitochondrial outer membrane permeabilization, BCL-2 family proteins, cytochrome c release, apoptosome formation, caspase-9 activation, and downstream executioner caspase-3/7 activation. The extrinsic apoptosis pathway is initiated by death receptors such as Fas, TNFR, and TRAIL receptors, which recruit adaptor proteins and activate caspase-8 before engaging executioner caspases or mitochondrial amplification through BID cleavage. Apoptosis is linked to many phenotypes, including cancer cell killing, tissue homeostasis, immune regulation, neurodegeneration, infection response, and treatment-induced cytotoxicity; unresolved questions include how apoptosis interacts with necroptosis, pyroptosis, ferroptos

Gram staining of tissue sections is a histochemical technique used to differentiate Gram-positive and Gram-negative bacteria within histological specimens based on differences in bacterial cell wall structure and dye retention, adapted from classical bacteriological Gram staining into tissue-compatible “histological Gram stain” variants. In tissue applications, modifications of the Brown-Hopps and Brown-Brenn methods are commonly used to improve differentiation of microorganisms embedded within host connective tissue and to reduce overstaining or loss of Gram-negative signal, which are known limitations of earlier approaches. The principle relies on crystal violet-iodine complex retention in Gram-positive organisms and subsequent decolorization and counterstaining steps that allow contrast visualization of Gram-negative organisms against tissue background.

The LIVE/DEAD bacterial viability staining method is based on differential permeability of nucleic-acid-binding fluorescent dyes, most commonly SYTO 9 and propidium iodide (PI), which enables discrimination of bacterial populations with intact versus compromised cytoplasmic membranes. SYTO 9 penetrates both intact and damaged bacterial membranes and binds nucleic acids to produce green fluorescence, whereas propidium iodide penetrates only cells with compromised membranes and fluoresces red while also reducing SYTO 9 signal through competitive binding and fluorescence interactions. The resulting fluorescence pattern is interpreted as a proxy for membrane integrity, which is widely used as an indicator of bacterial viability in microscopy, flow cytometry, and spectroscopic platforms. However, mechanistic studies show that SYTO 9 and PI interactions involve displacement and fluorescence resonance energy transfer effects, which can influence signal interpretation depending on dye ratios a