Microbial Morphological Identification: Observation Systems, Staining Methods, and Result Interpretation

Microbial morphological identification is a fundamental step in microbial classification, sample screening, contamination assessment, and experimental quality control. Its core purpose is not simply to observe “shape,” but to integrate cellular morphology, arrangement, staining reactions, special structures, colony characteristics, and culture-dependent morphological changes to preliminarily distinguish bacteria, fungi, actinomycetes, and other microorganisms. Morphological results usually cannot independently achieve species-level identification, but they provide important guidance for subsequent culture, biochemical identification, molecular testing, and antimicrobial susceptibility analysis.

Keywords: microbial morphological identification; bacterial morphology; fungal morphology; Gram staining; acid-fast staining; spore staining; capsule staining; colony morphology; microscopic observation; microbial quality control

1、Basic Logic of Microbial Morphological Identification

1.1 Observation Levels in Morphological Identification

(1) Cellular level

The cellular level mainly focuses on the shape, size, arrangement, and staining characteristics of individual microbial cells. Bacteria may appear as cocci, bacilli, curved rods, spirals, or filaments; fungi may appear as yeast-like cells, pseudohyphae, true hyphae, and spore structures. This level is suitable for rapid judgment of broad microbial categories and contamination types.

(2) Structural level

The structural level focuses on whether microorganisms possess special structures such as spores, capsules, flagella, branching hyphae, sporangia, conidia, or chlamydospores. Special structures often have relatively high differential value. For example, spores suggest directions related to Bacillus or Clostridium, while acid-fast positivity suggests Mycobacterium or some weakly acid-fast microorganisms.

(3) Population level

The population level mainly observes colony morphology, including colony size, color, transparency, margin, elevation, surface texture, viscosity, spreading behavior, hemolytic characteristics, and pigment production. Colony morphology is strongly affected by culture medium, temperature, incubation time, and oxygen conditions, and should be interpreted under standardized conditions.

(4) Dynamic level

The dynamic level focuses on motility, growth rate, morphological conversion, and stage-dependent changes during culture. Some bacteria show flagellar motility, some fungi switch between yeast and hyphal forms, and certain microorganisms form dormant structures under nutrient limitation or stress conditions.

1.2 Application Value of Morphological Identification

(1) Rapid preliminary screening

Morphological observation can quickly determine whether a sample contains bacteria, yeast-like fungi, filamentous fungi, or mixed contamination. It has high immediate value in culture contamination checks, clinical sample screening, fermentation process monitoring, and environmental microbial testing.

(2) Guidance for identification direction

Morphological features such as clustered Gram-positive cocci, Gram-negative rods, acid-fast-positive rods, spore-forming rods, and pseudohyphae-positive yeast-like fungi can guide the subsequent selection of culture media, biochemical tests, mass spectrometry identification, or molecular detection.

(3) Quality control

In microbiological experiments, morphological identification can be used to assess strain purity, stability after passage, culture contamination, and staining-process quality control. If multiple morphologies or inconsistent staining reactions appear in the same culture, mixed culture, contamination, or abnormal culture status should be considered first.

Table 1 Main Observation Levels in Microbial Morphological Identification

Observation Level	Observation Content	Representative Information	Main Use
Cellular level	Shape, size, arrangement, staining reaction	Cocci, rods, yeast-like cells, hyphae	Rapid judgment of broad microbial category
Structural level	Spores, capsules, flagella, hyphae, spores	Spore formation, acid-fastness, capsule positivity	Indication of specific groups or pathogenicity-related features
Population level	Colony morphology, pigments, hemolysis, texture	Smooth type, rough type, mucoid type, fluffy type	Assists purity assessment and preliminary classification
Dynamic level	Motility, growth rate, morphological conversion	Flagellar motility, dimorphism, hyphal formation	Assists identification and culture-status assessment

2、Bacterial Morphological Identification

2.1 Cellular Morphology

(1) Cocci

Cocci are usually round or nearly round and can be further distinguished by their arrangement. Grape-like clusters often suggest Staphylococcus-like morphology, chain-like arrangements are common in Streptococcus-like morphology, paired arrangements may be seen in diplococcus-like morphology, and tetrads or sarcina-like arrangements also have certain differential value. Arrangement should be interpreted together with smear quality and culture stage, and cannot alone serve as the final basis for identification.

(2) Bacilli

Bacilli may appear as short rods, long rods, thick rods, thin rods, coccobacilli, or spindle-shaped forms. Some rods have rounded ends, while others have square ends or club-shaped forms. Some cells may show palisade, V-shaped, or irregular arrangements. Rod morphology is strongly affected by culture time, and older cultures may show pleomorphism or uneven staining.

(3) Vibrios and spirilla

Vibrios often appear as curved comma-shaped or short curved rods, while spirilla and spirochetes show more obvious curved or spiral structures. These microorganisms require higher microscopic resolution, wet-mount observation, or special staining. Ordinary Gram staining may not fully display their details.

(4) Filamentous and branching bacteria

Actinomycete-like bacteria, Nocardia-like bacteria, and some environmental bacteria may appear filamentous or branching. These morphologies are easily confused with fungal hyphae, but bacterial filaments are usually thinner and differ in staining reaction, acid-fastness, and culture characteristics.

2.2 Gram Staining Characteristics

(1) Gram-positive bacteria

Gram-positive bacteria usually appear purple or blue-purple because of their thick peptidoglycan cell wall layer. Cocci, bacilli, and some filamentous bacteria may all be Gram-positive. If decolorization is excessive or the culture is aged, Gram-positive bacteria may show false-negative or uneven staining.

(2) Gram-negative bacteria

Gram-negative bacteria usually appear red or pink. Their outer membrane structure and cell wall characteristics make them less able to retain crystal violet. Most Enterobacteriaceae, non-fermenters, and vibrios appear as Gram-negative rods or curved rods.

(3) Gram-variable bacteria

Some bacteria may show Gram variability at different growth stages or under different processing conditions. Aged cultures, antimicrobial exposure, cell wall damage, or unstable staining procedures can all cause mixed staining patterns in the same strain.

Table 2 Interpretation Points for Bacterial Morphology and Gram Staining

Morphological Category	Typical Microscopic Appearance	Common Arrangement	Interpretation Focus
Gram-positive cocci	Purple round cells	Clusters, chains, pairs	Interpret with arrangement and colony morphology
Gram-negative cocci	Red or pink round cells	Often paired	Pay attention to smear thickness and decolorization
Gram-positive rods	Purple rod-shaped cells	Single, chain-like, palisade	Observe for spores or club-shaped morphology
Gram-negative rods	Red rod-shaped cells	Single or short chains	Interpret with colony, biochemical, and molecular tests
Curved rods/vibrios	Curved or comma-shaped	Mostly single	Ordinary smears may not show details sufficiently
Branching filamentous bacteria	Fine filamentous or branching	Hypha-like network	Must be distinguished from fungal hyphae

2.3 Observation of Special Structures

(1) Spores

Spores are highly resistant structures formed by some bacteria under unfavorable conditions. Spore position may be central, subterminal, or terminal; shape may be round or oval; and whether the spore causes swelling of the bacterial cell also has differential value. Spore staining is often used to distinguish spore-forming bacteria from non-spore-forming bacteria.

(2) Capsules

Capsules are extracellular polysaccharide- or polypeptide-like structures in some bacteria and can enhance antiphagocytic ability or environmental adaptability. Capsules are usually not easily stained directly by ordinary dyes and are often shown as clear halos around cells by negative staining or capsule staining.

(3) Flagella

Flagella are associated with bacterial motility and may be polar, bipolar, peritrichous, or tufted. Because flagella are very thin, they are difficult to observe directly under ordinary light microscopy. Flagellar staining, semisolid medium, or wet-mount motility observation is used for indirect assessment.

(4) Acid-fast structures

Mycobacteria and related organisms contain abundant lipid components in their cell walls and are not easily stained by ordinary methods, but can appear acid-fast-positive after acid-fast staining. Weakly acid-fast microorganisms may also show varying acid-fastness under specific staining conditions.

Table 3 Bacterial Special Structures and Their Differential Significance

Special Structure	Common Observation Method	Positive Appearance	Differential Significance
Spores	Spore staining	Spores and vegetative cells appear in different colors	Suggests spore-forming bacteria
Capsules	Negative staining, capsule staining	Clear halo around cells	Indicates capsule-forming ability
Flagella	Flagellar staining, motility test	Flagellar structure or spreading growth	Determines motility and flagellar type
Acid-fastness	Acid-fast staining	Red or bright-red acid-fast-positive rods	Suggests mycobacteria or weakly acid-fast microorganisms
Metachromatic granules	Special staining	Deeply stained granules	Assists observation of Corynebacterium-like groups

3、Fungal Morphological Identification

3.1 Yeast-Like Fungi

(1) Yeast cells

Yeast-like fungi are usually round, oval, or elliptical and commonly reproduce by budding. Under the microscope, budding cells, the connection between mother and daughter cells, and whether cell size is uniform can be observed.

(2) Pseudohyphae

Pseudohyphae are formed when yeast-like cells continue budding but do not fully separate, leaving constricted junctions between cells. Pseudohyphae are commonly observed in Candida-like fungal identification, but their presence depends on culture medium, temperature, and incubation time.

(3) True hyphae

Some yeast-like fungi can form true hyphae or germ tubes under specific conditions. Germ tube tests and morphology on cornmeal agar can be used for preliminary identification of yeast-like fungi.

3.2 Filamentous Fungi

(1) Hyphal morphology

Hyphae of filamentous fungi can be septate or aseptate/pauciseptate. Hyphal width, branching angle, presence or absence of septa, and transparency all have differential value. Broad, aseptate hyphae often suggest zygomycete-like fungi, while septate hyphae are commonly seen in Aspergillus, Penicillium, and related groups.

(2) Spore structures

Identification of filamentous fungi often depends on spore production patterns and spore morphology. Conidia, sporangiospores, chlamydospores, and arthroconidia can provide important clues for preliminary genus-level identification.

(3) Sporulating structures

Aspergillus-like fungi may show conidial heads, Penicillium-like fungi may show brush-like branching, and Mucor-like fungi may show sporangia and rhizoids. The integrity of sporulating structures is affected by culture conditions, and improper slide preparation can destroy key structures.

3.3 Fungal Colony Morphology

(1) Yeast-like colonies

Yeast-like colonies are often moist, smooth, creamy, or mucoid, and may appear white, off-white, pink, or other colors. Colony morphology can be used for screening, but there is overlap among different species.

(2) Filamentous fungal colonies

Filamentous fungal colonies are often fluffy, cottony, powdery, granular, or felt-like. Colony surface color, reverse pigmentation, spreading rate, and margin morphology can all provide identification clues.

(3) Influence of culture conditions

Fungal colony morphology is highly dependent on medium, temperature, humidity, incubation time, and light exposure. During morphological identification, the medium type and incubation time should be recorded; otherwise, results from different batches are not comparable.

Table 4 Main Observation Indicators for Fungal Morphological Identification

Fungal Type	Microscopic Observation Focus	Colony Observation Focus	Differential Significance
Yeast-like fungi	Budding, pseudohyphae, germ tubes	Moist, smooth, creamy	Preliminary distinction of yeast-like groups
Candida-like fungi	Blastoconidia, pseudohyphae, chlamydospores	White to off-white colonies	Assists judgment of Candida-like morphology
Aspergillus-like fungi	Septate hyphae, conidial heads	Powdery or fluffy, characteristic colors	Identification depends on sporulating structures
Penicillium-like fungi	Septate hyphae, brush-like branching	Blue-green or gray-green powdery colonies	Observe branching pattern and spore arrangement
Mucor-like fungi	Broad pauciseptate hyphae, sporangia	Rapid growth, cottony colonies	Must be distinguished from other filamentous fungi
Dermatophytes	Macroconidia, microconidia	Powdery, fluffy, or waxy colonies	Requires integration with hair/skin-scale samples and culture features

4、Actinomycetes and Other Microorganisms

4.1 Actinomycete-Like Microorganisms

(1) Branching filamentous morphology

Actinomycete-like microorganisms can form thin branching filamentous structures and are easily confused with fungal hyphae. Compared with fungi, actinomycete filaments are usually thinner and show bacterial-like staining characteristics.

(2) Culture characteristics

Actinomycete colonies are often dry, rough, and tough, and may produce an earthy odor or pigments. Some species grow slowly and may not form typical colonies if incubation time is insufficient.

(3) Staining assistance

Some actinomycete-like microorganisms may appear as Gram-positive branching rods, while some Nocardia species may be weakly acid-fast-positive. Combining Gram staining, acid-fast staining, and culture morphology can improve preliminary screening accuracy.

4.2 Spirochetes and Special-Morphology Microorganisms

(1) Spirochetes

Spirochetes are very thin and difficult to observe under ordinary light microscopy. Dark-field microscopy, silver staining, or immunofluorescence is often required. Their morphological features include slender spirals, bending motility, and high flexibility.

(2) Mycoplasma

Mycoplasma lacks a cell wall, so ordinary Gram staining is not suitable as the main observation method. Its colonies may show a characteristic “fried-egg” appearance, but this requires specific culture conditions and prolonged incubation.

(3) Chlamydia and Rickettsia

Chlamydia and Rickettsia are obligate intracellular or special-growth microorganisms. Conventional morphological observation has limited value, and confirmation usually requires cell culture, immunostaining, or molecular testing.

Table 5 Morphological Identification Points for Actinomycetes and Special Microorganisms

Microorganism Type	Main Morphological Features	Common Observation Methods	Interpretation Notes
Actinomycetes	Fine filamentous and branching	Gram staining, culture observation	Easily confused with fungal hyphae
Nocardia-like microorganisms	Branching rods, may be weakly acid-fast	Gram staining, weak acid-fast staining	Acid-fastness may be uneven
Spirochetes	Slender spiral morphology	Dark-field, silver staining, immunostaining	Ordinary staining has low sensitivity
Mycoplasma	No cell wall, special colonies	Special medium observation	Ordinary Gram staining has limited significance
Chlamydia/Rickettsia	Intracellular parasitism	Cell culture, immunostaining, molecular testing	Morphology alone cannot confirm identification

5、Colony Morphological Identification

5.1 Colony Morphology Indicators

(1) Size and shape

Colony size is affected by growth rate, incubation time, and nutritional conditions. Shapes may be circular, irregular, rhizoid, or spreading. Fast-growing organisms usually form obvious colonies earlier, while slow-growing organisms may require extended incubation.

(2) Margin and elevation

Colony margins may be entire, undulate, lobate, filamentous, or rhizoid. Elevation may be flat, slightly raised, convex, umbonate, or centrally depressed. Margin and elevation help distinguish colony phenotypes but should be compared under the same culture conditions.

(3) Surface and texture

Colony surfaces may be smooth, rough, wrinkled, moist, dry, mucoid, powdery, or fluffy. Bacterial colonies often appear smooth or rough, while fungal colonies commonly appear fluffy, powdery, or cottony.

(4) Color and pigment

Some microorganisms produce water-soluble or non-water-soluble pigments. During observation, the colony color itself should be distinguished from diffusible pigment in the medium. Pigment production is strongly affected by temperature, light, and medium composition.

Table 6 Colony Morphology Observation Indicators and Their Significance

Observation Indicator	Common Appearance	Differential Significance	Notes
Size	Pinpoint, small, medium, large	Reflects growth rate and culture status	Incubation time must be fixed
Margin	Entire, undulate, lobate, filamentous	Assists colony phenotype distinction	Affected by inoculum size
Elevation	Flat, convex, umbonate, depressed	Assists colony characterization	Best observed with side lighting
Surface	Smooth, rough, wrinkled, mucoid	Reflects extracellular structure or population features	Older cultures may change
Texture	Moist, dry, powdery, fluffy	Distinguishes bacteria, yeasts, and molds	Fungi require microscopic structural confirmation
Pigment	Colony color or diffusible pigment	Helps identify certain microbial groups	Affected by medium and temperature
Hemolysis	Alpha, beta, gamma hemolysis	Assists bacterial screening	Requires standardized blood agar and culture conditions

6、Common Staining Methods and Selection

6.1 Basic Staining Methods

(1) Simple staining

Simple staining uses a single dye to display cellular morphology and is suitable for observing bacterial shape, size, and arrangement. The method is simple but does not provide Gram characteristics, acid-fastness, or special-structure information.

(2) Gram staining

Gram staining is the most commonly used method in bacterial morphological identification. It provides information on cellular morphology, arrangement, and Gram reaction. Result quality depends on smear thickness, fixation method, decolorization time, and culture freshness.

(3) Wet-mount observation

Wet mounts can be used to observe motility, yeast budding, fungal hyphae, and some protozoan morphology. This method preserves sample viability but requires operator experience and careful microscope adjustment.

6.2 Special Staining Methods

(1) Acid-fast staining

Acid-fast staining is suitable for observing mycobacteria and some weakly acid-fast microorganisms. Acid-fast-positive organisms usually appear red against a blue or green background. Weakly acid-fast microorganisms require modified acid-fast staining conditions.

(2) Spore staining

Spore staining is used to display spore-forming bacteria. In typical results, spores and vegetative cells show different colors, allowing observation of spore position, shape, and whether the spore causes cell swelling.

(3) Capsule staining

Capsule staining or negative staining can show clear zones around cells and is suitable for observing capsule-forming ability. Excessive heating of smears may destroy capsule structures, so smear preparation should be gentle.

(4) Lactophenol cotton blue staining

Lactophenol cotton blue is commonly used for microscopic observation of filamentous fungal morphology and can display hyphae and spore structures. During slide preparation, sporulating structures should be preserved as much as possible, and excessive pressing should be avoided.

Table 7 Common Staining Methods for Microbial Morphological Identification

Staining Method	Main Applicable Objects	Observation Focus	Limitations
Simple staining	Bacteria, yeasts	Cellular morphology and arrangement	Cannot distinguish Gram reaction
Gram staining	Most bacteria, some fungi	Gram reaction, morphology, arrangement	Affected by decolorization and culture status
Acid-fast staining	Mycobacteria, weakly acid-fast bacteria	Acid-fast-positive rods	A negative ordinary result cannot fully exclude them
Spore staining	Spore-forming bacteria	Spore position and morphology	Requires heating or enhanced staining conditions
Capsule staining	Capsule-forming bacteria	Capsule around cells	Excessive preparation may affect results
Flagellar staining	Motile bacteria	Number and distribution of flagella	Technically demanding
Lactophenol cotton blue staining	Filamentous fungi	Hyphae, spores, sporulating structures	Pressing may destroy structures
India ink negative staining	Encapsulated yeasts, etc.	Clear capsule-like structures	Background quality affects interpretation

7、Quality Control in Morphological Identification

7.1 Control of Samples and Culture Conditions

(1) Incubation time

Microbial morphology changes markedly with growth stage. Aged bacterial cultures may show pleomorphism, unstable Gram staining, or autolysis; fungal cultures may not form typical sporulating structures if incubation time is insufficient.

(2) Culture medium type

Different media affect colony size, pigmentation, mucoid appearance, and fungal sporulation. Morphological comparisons should be performed on the same medium, at the same temperature, and for the same incubation time whenever possible.

(3) Inoculum size

Excessive inoculum can cause colony fusion and affect single-colony observation; insufficient inoculum may delay growth and produce atypical morphology. Isolated colonies should be obtained first for pure-culture isolation.

7.2 Smear and Staining Control

(1) Smear thickness

A smear that is too thick may cause uneven decolorization, cell overlap, and unclear morphology; a smear that is too thin may contain too few organisms. An ideal smear should be uniform, thin, and representative.

(2) Fixation conditions

Excessive heat fixation can cause cell deformation, rupture, or loss of structures; insufficient fixation may cause smear detachment. Suitable fixation methods should be selected according to sample type.

(3) Staining time

Staining, decolorization, and counterstaining times must be consistent. Decolorization is especially critical in Gram staining: over-decolorization may cause false-negative Gram-positive results, while under-decolorization may cause false-positive Gram-negative results.

7.3 Interpretation Control

(1) Positive controls

Morphological staining should include typical positive controls, such as Gram-positive bacteria, Gram-negative bacteria, spore-positive bacteria, acid-fast-positive bacteria, or known fungal cultures, to confirm that the staining system is working.

(2) Negative controls

In special staining, negative controls can help exclude background staining, nonspecific deposits, and procedural contamination. Acid-fast staining, capsule staining, and fungal staining especially require careful background interpretation.

(3) Multi-method validation

Morphological results are usually screening results. For important samples, they should be confirmed by culture purification, biochemical identification, mass spectrometry, molecular testing, serology, or sequencing.

Table 8 Common Problems and Optimization Strategies in Microbial Morphological Identification

Problem	Possible Cause	Effect on Result	Optimization Strategy
Unclear Gram reaction	Smear too thick, improper decolorization, aged culture	Misjudgment of Gram reaction	Use fresh culture and control decolorization time
Cell morphology deformation	Excessive fixation, rapid drying	Atypical morphology	Optimize fixation and smear thickness
Unclear fungal sporulating structures	Insufficient incubation or compression damage	Difficulty in genus-level judgment	Extend incubation and use tape mount or slide culture
Unstable colony characteristics	Different medium, temperature, or incubation time	Poor comparability between groups	Standardize culture conditions
Strong background staining	Dye precipitation, insufficient washing	Structures difficult to recognize	Filter staining solution and wash thoroughly
Multiple morphologies in one sample	Mixed culture, contamination, or complex sample	Difficult result interpretation	Re-isolate pure culture and recheck
Special structures not displayed	Incompatible staining method or excessive processing	Risk of false negative	Use dedicated staining and positive controls

8、Result Interpretation and Application Boundaries

8.1 Principles for Interpreting Morphological Results

(1) Morphology is preliminary identification

Microbial morphological identification can provide broad category judgment and guide identification direction, but in most cases it cannot directly achieve species-level identification. Different microorganisms with similar morphology may have completely different biochemical features, genetic backgrounds, and pathogenic significance.

(2) A single feature cannot independently define the result

Gram positivity, spherical shape, rod shape, mucoid colonies, pigment production, or sporulating structures all need to be interpreted with additional indicators. A single morphological feature should only be used as a clue and not as a final conclusion.

(3) Morphology must be linked to culture background

The same microorganism may show different appearances under different media, temperatures, oxygen environments, and incubation times. When reporting morphological results, sample source, culture conditions, staining method, and observation magnification should be recorded.

8.2 Application Boundaries in Different Scenarios

(1) Clinical samples

Morphological observation of clinical samples can rapidly suggest infection type, but must be combined with culture, antimicrobial susceptibility testing, and molecular detection. Microbial morphology observed in direct smears may be affected by antimicrobial exposure, host cells, and sample processing.

(2) Industrial fermentation

In fermentation systems, morphological identification can be used to monitor contamination, cell status, and production-strain stability. If non-target morphology, abnormal motility, or mixed colony appearance occurs, purification and molecular confirmation should be performed promptly.

(3) Laboratory quality control

Strains should be morphologically rechecked after preservation, passage, and recovery. If colony morphology, Gram reaction, or microscopic structures differ from the standard strain, contamination, mutation, degeneration, or labeling error should be considered.

Table 9 Application Boundaries of Microbial Morphological Identification Results

Application Scenario	Information Provided by Morphology	Limitations	Subsequent Validation
Sample screening	Determines presence of bacteria, fungi, or mixed microorganisms	Cannot directly identify species	Culture, biochemical, molecular testing
Contamination investigation	Determines whether contaminating bacteria or fungi are present	Cannot confirm contamination source	Isolation culture, environmental monitoring
Strain quality control	Compares colony, staining, and microscopic morphology	Normal morphology does not equal genetic stability	Mass spectrometry, sequencing, functional testing
Clinical indication	Rapidly suggests infection type	Easily affected by sample quality	Culture, antimicrobial susceptibility testing, nucleic acid detection
Fermentation monitoring	Observes production strain status and contamination risk	Cannot quantitatively evaluate all contamination	Plate count, qPCR, metabolic analysis
Teaching and training	Builds morphological recognition ability	Relatively subjective	Standard atlases and blinded sample assessment

9、Product Selection for Microbial Morphological Identification

Table 10 Selection of Staining and Slide Preparation Reagents for Microbial Morphological Identification

Method Module	Product Category	Product Name	CAS No.	Role in System	Applicable Direction
Gram staining	Primary stain	Crystal violet	548-62-9	Primary stain in Gram staining; makes Gram-positive bacteria appear purple or blue-purple	Observation of bacterial Gram reaction, morphology, and arrangement
Gram staining	Mordant auxiliary reagent	Potassium iodide	7681-11-0	Improves iodine solubility in aqueous phase and stabilizes Gram iodine solution	Preparation of Gram iodine solution
Gram staining	Counterstain	Safranin O	477-73-6	Counterstains decolorized Gram-negative bacteria	Observation of Gram-negative bacteria and mixed flora
Gram staining/counterstaining	Counterstain	Basic fuchsin	632-99-5	Can be used for counterstaining or enhanced bacterial visualization	Bacterial morphology observation and acid-fast staining-related systems
Acid-fast staining	Staining enhancer component	Phenol	108-95-2	Enhances fuchsin penetration into lipid-rich cell walls	Carbol fuchsin system
Acid-fast staining	Counterstain	Methylene blue	61-73-4	Shows background cells and non-acid-fast bacteria	Acid-fast counterstaining and ordinary bacterial observation
Spore staining	Spore primary stain	Malachite green	569-64-2	Displays spore structures and contrasts them with vegetative cells	Observation of Bacillus-like and Clostridium-like spores
Albert/metachromatic granule staining	Background dye	Malachite green oxalate	2437-29-8	Shows bacterial background and outline, enhancing granule contrast	Observation of metachromatic granules in Corynebacterium-like bacteria
Metachromatic granule staining	Metachromatic granule dye	Toluidine blue O	92-31-9	Shows metachromatic granules or acidic granule structures	Albert staining and Corynebacterium-like morphology observation
Neisser staining	Background dye	Bismarck brown Y	10114-58-6	Produces bacterial background color and enhances granule contrast	Background display in Neisser staining
Staining system adjustment	Acidity regulator	Glacial acetic acid	64-19-7	Adjusts acidic conditions in staining system	Albert stain or related special staining systems
Smear preparation	Fixation auxiliary reagent	Methanol	67-56-1	Fixes bacterial smears and reduces cell detachment and morphological damage	Pre-staining fixation treatment
Fungal wet mount	Clearing reagent	Potassium hydroxide	1310-58-3	Digests keratin and tissue background, highlighting fungal hyphae or spores	Microscopic examination of skin scales, hair, and nail debris
Fungal staining	Fungal structural dye	Aniline blue / cotton blue	28631-66-5	Displays fungal hyphae and spore structures	Lactophenol cotton blue staining
Fungal staining	Lactophenol system component	Lactic acid	50-21-5	Preserves and clears fungal structures, improving microscopic background	Lactophenol cotton blue system
Fungal staining/slide preparation	Moisturizing and refractive adjustment reagent	Glycerol	56-81-5	Keeps mounts moist and improves refraction and preservation	Fungal wet mounts and temporary preparations
Capsule/negative staining	Negative stain	Congo red	573-58-0	Forms background contrast and assists observation of capsule or boundaries	Negative staining and capsule-related observation
Microscopic observation	Oil immersion medium	Cedarwood oil	8000-27-9	Improves resolution and image clarity under 100× oil objective	Oil immersion observation of bacterial stained smears

Microbial morphological identification should be based on standardized culture, proper smear preparation, correct staining, and multi-level observation. Bacterial identification focuses on cellular morphology, arrangement, Gram reaction, and special structures; fungal identification focuses on yeast-like structures, hyphae, spores, and colony characteristics; actinomycetes and special microorganisms require interpretation combined with special staining or culture conditions.

For more related articles, please see below:

[1] Principles and methods of smear staining, microbiological staining, and fundamental dye systems

[2] Biological Stain